with reference to the drawings , embodiments of the present invention will be described in detail by taking an optical disk for example . fig1 is a block diagram showing a structure of an image display apparatus according to a first embodiment of the present invention . fig2 is a flowchart showing an operation of this image display apparatus . the image display apparatus includes an optical disk drive 2 for reading data from an optical disk 1 , an mpeg decoder 3 for decoding mpeg encoded audio data and moving image data , an image memory 4 for storing image data decoded by the mpeg decoder 3 , a display controller 6 for causing a display device 5 to display image data stored in the image memory 4 , a rom 7 for storing an image display program , a cpu 8 for controlling the apparatus in accordance with the program , a ram 9 for storing an address , etc ., of moving image data , and an input device 10 used by a user for giving an instruction . with reference to fig2 , an operation of the above - described image display apparatus will be described . first , when the optical disk 1 is mounted on the optical disk drive 2 , the cpu 8 causes the optical disk drive 2 to read a table stored in a predetermined location on the optical disk 1 . in this table , addresses ( minute , second , frame ) of all mpeg encoded moving image data stored in the optical disk 1 are registered . then , the cpu 8 stores the obtained addresses of the moving image data in the ram 9 ( s 201 ). the cpu 8 retrieves a start address of the moving image data to be displayed from the ram 9 ( s 202 ), reads data from the optical disk 1 by moving an optical head of the optical disk drive 2 to a location of the start address of the moving image data to be displayed on the optical disk 1 ( s 203 ), and inputs the read data to the mpeg decoder 3 ( s 204 ). the mpeg decoder 3 decodes the mpeg encoded data . when an output of a frame following the start address of the moving image data to be displayed is detected , the cpu 8 reduces a size of the image data obtained as a result of decoding by the mpeg decoder 3 ( s 205 ), and writes the thumbnail image data generated as a result of a size reduction process to a predetermined memory area in the image memory 4 . further , the cpu 8 compares among color - difference components ( color - difference data ) of the pixels of the thumbnail image data written to the image memory 4 for determining whether or not there is a variation in the color - difference data ( that is , whether or not the thumbnail image data includes pixels having different color - difference data )( s 206 ). if there is no variation in the color - difference data , the cpu goes back to step s 205 , and performs the same process for a next frame . after repeating the processes at steps s 205 and s 206 , if determination is finally made at step s 206 that the thumbnail image data written to the image memory 4 includes pixels having different color - difference data , the cpu 8 instructs the display controller 6 to display a thumbnail image based on the above thumbnail image data on the display device 5 ( step s 207 ). as a result , as shown in fig3 a , a thumbnail image 31 is displayed on the display device 5 . the cpu 8 subsequently determines whether or not another thumbnail image is further displayed by checking , for example , whether or not there is moving image data whose thumbnail image has yet to be displayed in the optical disk 1 ( s 208 ). if it is determined that another thumbnail image is to be displayed , the cpu 8 goes back to step s 202 , and repeats the same process for moving image data to be displayed next . the thumbnail image data obtained by the above process is written to a memory area , which is different from the area to which the thumbnail image 31 was written , in the image memory 4 . as a result , as shown in fig3 b , a new thumbnail image 32 is displayed next to the thumbnail image 31 . by repeating the above - described operation , a digest screen as shown in fig3 c in which a plurality of thumbnail images are arranged is displayed . note that , in fig3 c , seven thumbnail images are displayed on the screen , but the present invention is not limited thereto . it will be understood that the number of thumbnail images displayed on the screen may be arbitrary . also , in the case where the number of thumbnail images generated from the moving image data stored in the optical disk 1 exceeds the maximum number of thumbnail images that can be displayed on one screen ( for example , in fig3 c , the maximum number is nine ), the cpu 8 may newly start displaying after deleting all displayed thumbnail images . alternatively , the cpu 8 may leave a portion of the displayed thumbnail images , and arrange newly generated thumbnail images next thereto . note that , at step s 206 , determination about whether or not there is a difference in the color - difference component may be made in consideration of all pixels of the thumbnail image . alternatively , the determination may be made in consideration of a portion of pixels ( e . g ., pixels near the center of the thumbnail image ) of the thumbnail image . if determination is made in consideration of a portion of the pixels , it is possible to reduce the processing load at step s 206 . note that , in the present embodiment , a thumbnail image is displayed if the thumbnail image includes pixels having different color - difference data . however , the present invention is not limited thereto . for example , a thumbnail image may be displayed only in the case where the thumbnail image includes three or more pixels having different color - difference data ( that is , in the case where the thumbnail image includes three or more colors ). similarly , a thumbnail image may be displayed only in the case where the thumbnail image includes four or more pixels having different color - difference data . thus , it is possible to display a further significant thumbnail image . as such , in the present embodiment , thumbnail images are sequentially generated from a head of the moving image , and a thumbnail image having different color - difference components is displayed as a digest , whereby it is possible to prevent a black or white image from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . fig4 is a flowchart showing an operation of an image display apparatus according to a second embodiment of the present invention . in fig4 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the second embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . in the first embodiment , comparison of a color difference is performed by referring to thumbnail image data . the present embodiment is characterized in that comparison of a color difference is performed by referring to image data for which a size reduction process has not been performed . when an output of a frame following a start address of moving image data to be displayed is detected ( s 401 ), the cpu 8 compares a color - difference component of each pixel of the image data of this frame for determining whether or not there is a variation in color - difference data in this image ( s 402 ). if there is no variation in the color - difference data , the cpu 8 goes back to step s 401 , and performs the same process for a next frame . after repeating the processes at steps s 401 and s 402 , if determination is finally made at step s 402 that a piece of image data includes pixels having different color - difference data , the cpu 8 reduces a size of the image data ( s 403 ), and writes the thumbnail image generated as a result of a size reduction process to a predetermined memory area in the image memory 4 . then , at step s 207 , a thumbnail image is displayed based on the above - described thumbnail image data . note that , at step s 402 , determination about whether or not there is a difference in the color - difference component may be made in consideration of all pixels of the image . alternatively , the determination may be made in consideration of a portion of pixels ( e . g ., pixels near the center of the thumbnail image ) of the image . if determination is made in consideration of a portion of the pixels , it is possible to reduce the processing load at step s 402 . in the present embodiment , it is assumed that a thumbnail image corresponding to an image including pixels having different color - difference data is displayed . however , the present invention is not limited thereto . for example , only in the case where an image includes three or more pixels having different color - difference data ( that is , in the case where the image includes three or more colors ), a thumbnail image corresponding to the image may be displayed . similarly , only in the case where an image includes four or more pixels having different color - difference data , a thumbnail image corresponding to the image may be displayed . thus , it is possible to display a further significant thumbnail image . as such , in the present embodiment , an image having different color - difference components is retrieved from a head of the moving image , and a thumbnail image corresponding to the image is displayed as a digest , whereby it is possible to prevent a black or white image from being displayed as a digest . as a result , it is possible to display a digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . note that , compared to the second embodiment , the above - mentioned first embodiment has the following advantages : comparison of a color difference of each pixel requires a smaller processing load since the significance of an image is determined based on a thumbnail image whose size is smaller than an original image ; and more precise determination results can be obtained since the significance of an image is determined based on a thumbnail image actually displayed on the screen . fig5 is a flowchart showing an operation of an image display apparatus according to a third embodiment of the present invention . in fig5 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the third embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . in the first embodiment , a color difference is compared among pixels of the same thumbnail image data . the present embodiment is characterized in that a color difference is compared between two pieces of thumbnail image data on a pixel basis . when an output of a frame following a start address of moving image data to be displayed is detected , the cpu 8 reduces a size of the image data obtained as a result of decoding by the mpeg decoder 3 ( s 501 ), and writes the thumbnail image data ( that is , thumbnail image data corresponding to a first frame ; hereinafter , referred to as first thumbnail image data ) generated as a result of a size reduction process to a predetermined memory area in the image memory 4 . further , the cpu 8 detects an output of a next frame , performs a size reduction process for image data of this frame , and writes the thumbnail image data ( hereinafter , referred to as second thumbnail image data ) generated as a result of a size reduction process to a predetermined memory area in the image memory 4 ( s 502 ). subsequently , the cpu 8 compares a color - difference component ( color - difference data ) between the first thumbnail image data and the second thumbnail image data on a pixel basis for determining whether or not there is a difference in the color - difference data ( that is , there is a pixel whose color - difference data is different between the first thumbnail image data and the second thumbnail image data ) ( s 503 ). if there is no difference in the color - difference data , the cpu 8 goes back to step s 502 , and performs the same process using thumbnail image data of a next frame as second thumbnail image data . after repeating the processes at steps s 502 and s 503 , if determination is finally made at step s 503 that there is a pixel whose color - difference data is different between the first thumbnail image data and the second thumbnail image data , the cpu 8 instructs the display controller 6 to display a thumbnail image based on the second thumbnail image data on the display device 5 ( s 504 ). note that , at step s 503 , determination about whether or not there is a difference in the color - difference component may be made in consideration of all pixels of each thumbnail image data . alternatively , the determination may be made in consideration of a portion of pixels ( e . g ., pixels near the center of the thumbnail image ) of each thumbnail image data . if determination is made in consideration of a portion of the pixels , it is possible to reduce the processing load at step s 503 . also , in the present embodiment , a thumbnail image of a first frame is used as first thumbnail image data . however , the present invention is not limited thereto . for example , a thumbnail image of a frame immediately before the second thumbnail image data may be sequentially used as first thumbnail image data for comparing a color - difference component between the frame and a next frame . as such , in the present embodiment , first thumbnail image data whose color - difference component is different from thumbnail image data of ahead of the moving image is retrieved , and a thumbnail image corresponding to the thumbnail image data is displayed as a digest , whereby it is possible to prevent a black or white image from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . fig6 is a flowchart showing an operation of an image display apparatus according to a fourth embodiment of the present invention . in fig6 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the fourth embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . in the third embodiment , comparison of a color difference is performed by referring to two pieces of thumbnail image data . the present embodiment is characterized in that comparison of a color difference is performed by referring to two pieces of image data for which a size reduction process has not been performed . when an output of a frame following a start address of the moving image data to be displayed is detected , the cpu 8 writes the image data ( that is , image data corresponding to a first frame ; hereinafter , referred to as first image data ) obtained as a result of decoding by the mpeg decoder 3 to a predetermined memory area in the image memory 4 ( s 601 ). further , the cpu 8 detects an output of a next frame , and writes this image data ( hereinafter , referred to as second image data ) to a predetermined memory area in the image memory 4 ( s 602 ). subsequently , the cpu 8 compares a color - difference component ( color - difference data ) between the first image data and the second image data , which are written to the image memory 4 , on a pixel basis for determining whether or not there is a difference in the color - difference data ( that is , there is a pixel whose color - difference data is different between the first image data and the second image data ( s 603 ). if there is no difference in the color - difference data , the cpu 8 goes back to step s 602 , and performs the same process using image data of a next frame as second image data . after repeating the processes at steps s 602 and s 603 , if determination is finally made at step s 603 that there is a pixel whose color - difference data is different between the first image data and the second image data , the cpu 8 reduces a size of the second image data ( s 604 ), and writes the thumbnail image data generated as a result of a size reduction process to a predetermined memory area in the image memory 4 . then , at step s 207 , a thumbnail image is displayed based on the above - described thumbnail image data . note that , at step s 603 , determination about whether or not there is a difference in the color - difference component may be made in consideration of all pixels of each image data . alternatively , the determination may be made in consideration of a portion of pixels ( e . g ., pixels near the center of the image ) of each image data . if determination is made in consideration of a portion of the pixels , it is possible to reduce the processing load at step s 603 . also , in the present embodiment , an image of a first frame is used as first image data . however , the present invention is not limited thereto . for example , a thumbnail image of a frame immediately before the second image data may be sequentially used as first image data for comparing a color - difference component between the frame and a next frame . as such , in the present embodiment , first image data whose color - difference component is different from image data of a head of the moving image is retrieved , and a thumbnail image corresponding to the image data is displayed as a digest , whereby it is possible to prevent a black or white image from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . note that , compared to the fourth embodiment , the above - mentioned third embodiment has the following advantages : comparison of a color difference of each pixel requires a smaller processing load since the significance of an image is determined based on a thumbnail image whose size is smaller than an original image ; and more precise determination results can be obtained since the significance of an image is determined based on a thumbnail image actually displayed on the screen . in the above - described first to fourth embodiments , determination about whether or not an image is significant is made based on a color difference . in place of color difference data , it is also effective to determine whether or not an image is significant based on brightness . in this case , “ a color difference ” in the first to fourth embodiments may be replaced with “ brightness ” for describing an operation of the image processing apparatus . by way of example , “ a color difference ” in the first embodiment is replaced with “ brightness ” for describing the first embodiment as a fifth embodiment . fig7 is a flowchart showing an operation of an image display apparatus according to the fifth embodiment of the present invention . in fig7 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the fifth embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . the cpu 8 compares among brightness components ( brightness data ) of the pixels of the thumbnail image data written to the image memory 4 for determining whether or not there is a variation in the brightness data ( that is , whether or not the thumbnail image data includes pixels having different brightness data ) ( s 701 ). if there is no variation in the brightness data , the cpu goes back to step s 205 , and performs the same process for a next frame . after repeating the processes at steps s 205 and s 701 , if determination is finally made at step s 701 that the thumbnail image data written to the image memory 4 includes pixels having different brightness data , the cpu 8 instructs the display controller 6 to display a thumbnail image based on the above thumbnail image data on the display device 5 ( step s 207 ). note that , at step s 701 , determination about whether or not there is a difference in the brightness component may be made in consideration of all pixels of the thumbnail image . alternatively , the determination may be made in consideration of a portion of pixels ( e . g ., pixels near the center of the thumbnail image ) of the thumbnail image . if determination is made in consideration of a portion of the pixels , it is possible to reduce the processing load at step s 701 . note that , in the present embodiment , a thumbnail image is displayed if the thumbnail image includes pixels having different brightness data . however , the present invention is not limited thereto . for example , a thumbnail image may be displayed only in the case where the thumbnail image includes three or more pixels having different brightness data . similarly , a thumbnail image may be displayed only in the case where the thumbnail image includes four or more pixels having different brightness data . thus , it is possible to display a further significant thumbnail image . as such , in the present embodiment , a thumbnail image is sequentially generated from a head of the moving image , and a thumbnail image having different brightness components is displayed as a digest , whereby it is possible to prevent a black or white image from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . note that “ a color difference ” in the second to fourth embodiments can be replaced with “ brightness ” in similar manners . in this case , a structure and an operation of the image processing apparatus would be obvious over the descriptions of each embodiment . fig8 is a flowchart showing an operation of an image display apparatus according to a sixth embodiment of the present invention . in fig8 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the sixth embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . when an output of a frame following a start address of the moving image data to be displayed is detected ( s 801 ), the cpu 8 calculates a bit rate of the image data obtained as a result of decoding by the mpeg decoder 3 , and determines whether or not the calculated bit rate is equal to or greater than a predetermined value ( s 802 ). if the calculated bit rate is smaller than a predetermined value ( that is , the image is a monotonous image ), the cpu 8 goes back to step s 801 for checking an output of a next frame and calculating a bit rate of the next frame . the cpu 8 repeats the above - described process . when image data whose bit rate is equal to or greater than a predetermined value is detected , the cup 8 reduces a size of the image data , and writes the thumbnail image data to a predetermined memory area in the image memory 4 ( s 803 ). then , at step s 207 , a thumbnail image is displayed based on the above - described thumbnail image data . as such , in the present embodiment , a frame whose bit rate is equal to or greater than a predetermined value is retrieved from a head of the moving image , and the retrieved frame is displayed as a digest , whereby it is possible to prevent a black or white image from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . fig9 is a flowchart showing an operation of an image display apparatus according to a seventh embodiment of the present invention . in fig9 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the seventh embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . when an output of a frame following a start address of the moving image data to be displayed is detected ( s 901 ), the cpu 8 calculates a bit rate of the image data ( which corresponds to a first frame ) obtained as a result of decoding by the mpeg decoder 3 , and stores the calculated bit rate ( s 902 ). assume that the above - described bit rate is ( a ). further , the cpu 8 detects an output of a next frame ( s 903 ), calculates a bit rate of image data ( hereinafter , referred to as second image data ) of this frame , and stores the calculated bit rate ( s 904 ). assume that the above - described bit rate is ( b ). then , the cpu 8 determines whether or not the bit rate ( a ) is equal to the bit rate ( b ) ( s 905 ). if the bit rate ( a ) is equal to the bit rate ( b ), the cpu 8 goes back to step s 903 , and performs the same process using a bit rate of a next frame as a bit rate ( b ). after repeating the processes from steps s 903 to s 905 , if a frame having a different bit rate is finally detected , the cpu 8 reduces a size of image data of the frame ( s 906 ). then , at step s 207 , a thumbnail image is displayed based on the above - described thumbnail image data . note that , in the present embodiment , the cpu 8 determines at step s 905 whether or not a bit rate ( a ) is equal to a bit rate ( b ), but the present invention is not limited thereto . for example , the cpu 8 may determine whether or not a difference between the bit rate ( a ) and the bit rate ( b ) is equal to or greater than a predetermined value . also , in the present embodiment , a bit rate of a first frame is used as a bit rate ( a ), but the present invention is not limit thereto . for example , a bit rate of a frame immediately before the second image data may be used as a bit rate ( a ) for performing comparison of a bit rate between the frame and a next frame . as such , in the present embodiment , a frame having a changed bit rate is retrieved from a head of the moving image , and the retrieved frame is displayed as a digest , whereby it is possible to prevent a black or white image from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . fig1 is a flowchart showing an operation of an image display apparatus according to an eighth embodiment of the present invention . in fig1 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the eighth embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . when an mpeg encoded audio data is detected in data following a start address ( s 1001 ), the cpu 8 detects an output of a frame after a time period represented by an address of the detected audio data , reduces a size of the frame ( s 205 ), and writes the thumbnail image data to a predetermined memory area in the image memory 4 . then , at step s 207 , a thumbnail image is displayed based on the above - described thumbnail image data . as such , in the present embodiment , a frame after a time period at which audio data is first detected is retrieved from a head of a moving image , and the retrieved frame is displayed as a digest , whereby it is possible to prevent a screen corresponding to a silent frame from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . fig1 is a flowchart showing an operation of an image display apparatus according to a ninth embodiment of the present invention . in fig1 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the ninth embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . when mpeg encoded caption data is detected in data following a start address ( s 1101 ), the cpu 8 detects a frame after a time period represented by an address of the detected caption data , reduces a size of the frame ( s 205 ), and writes the thumbnail image data to a predetermined memory area in the image memory 4 . then , at step s 207 , a thumbnail image is displayed based on the above - described thumbnail image data . as such , in the present embodiment , a frame after a time period at which caption data is detected is retrieved from a head of a moving image , and the retrieved frame is displayed as a digest , whereby it is possible to prevent a screen corresponding to a frame without speech in the moving image with the captions from being displayed as a digest . as a result , it is possible to display a significant digest screen allowing the user to easily grasp the contents of the moving image stored in the optical disk 1 . fig1 is a flowchart showing an operation of an image display apparatus according to a tenth embodiment of the present invention . in fig1 , any steps identical to their counterparts in fig2 are denoted by like step numbers , with the descriptions thereof omitted . the structure of the image display apparatus of the tenth embodiment is identical to its counterpart in the first embodiment ( in this embodiment , however , the cpu 8 executes a different image processing program ), and therefore fig1 is used in the following descriptions . the present embodiment corresponds to a combination of the above - described first embodiment and the eighth embodiment . when an mpeg encoded audio data is detected in data following a start address ( s 1001 ), the cpu 8 detects an output of a frame after a time period represented by an address of the detected audio data , reduces a size of the frame ( s 205 ), and writes the thumbnail image data to a predetermined memory area in the image memory 4 . subsequently , the cpu 8 compares among color - difference components of the pixels of the thumbnail image written to the image memory 4 for checking whether or not there is a variation in the color - difference data ( s 206 ). if there is no variation in the color - difference data , the cpu 8 goes back to step s 205 . after repeating the processes at steps s 205 and s 206 , the cpu 8 instructs the display controller 6 to display a thumbnail image including pixels having different color - difference data ( step s 207 ). as such , in the present embodiment , a plurality of methods for detecting a significant thumbnail image are combined , whereby it is possible to display a further significant thumbnail image . also , a process for generating a thumbnail image can be omitted until detection of audio data , whereby it is possible to reduce time required for retrieving a significant thumbnail image . note that the tenth embodiment is a combination of the first embodiment and the eighth embodiment , but a combination is not limited thereto . for example , three or more embodiments may be combined . note that , in the above - described first to tenth embodiments , a digest of mpeg encoded moving image data stored in the optical disk 1 is displayed , but the present invention is not limited thereto . for example , the present invention can be applied to a case in which a digest of moving image data stored in a recording medium other than the optical disk 1 ( e . g ., a hard disk ) is displayed . also , the present invention can be applied to a case in which a digest of moving image data other than mpeg encoded moving image data is displayed . also , in the above - described first to tenth embodiments , an image processing program is stored in the rom 7 . the image processing program may be stored in an arbitrary computer readable recording medium such as a cd - rom or a hard disk , or may be supplied to the image processing apparatus via a communication line . note that , in the first to tenth embodiments , a significant image is detected from a start address of each moving image . however , the present invention is not limited thereto . an address may be detected from a start address in reverse . as a result , in the case where the moving image is reproduced from the detected image , it is possible to reproduce desired data without skipping a start address thereof . while the invention has been described in detail , the foregoing description is in all aspects illustrative and not restrictive . it is understood that numerous other modifications and variations can be devised without departing from the scope of the invention .