an embodiment of the invention will be described with reference to the drawings . fig1 is a diagram showing a schematic structure of an image processing apparatus 10 according to a first embodiment , and fig2 is an explanatory diagram showing a raw file . in the image processing apparatus 10 , a cpu 12 for executing various calculating processings , an rom 14 for recording a program group to set hardware , an ram 16 for temporarily recording data when the cpu 12 executes various calculation processings , a clock generator 18 for generating an operation clock , a video board 20 including a display connector to which a display 40 is connected and an lan board 22 to which a network compatible printer 42 is connected through an lan cable are connected to an input / output controller 24 for performing a connection to various apparatuses and a control through a bus 26 . an input device 27 such as a keyboard or a mouse and a hard disk 28 having a large capacity are connected to the input / output controller 24 , and furthermore , a card type memory adaptor 32 capable of transferring data together with a card type memory 30 storing a raw file 50 ( see fig2 ) for an image photographed by means of a digital camera is connected thereto through a usb terminal . the hard disk 28 includes a program holder 28 a for storing various programs such as a jpeg file creating program for performing various corrections for raw data included in the raw file and creating a jpeg file . the raw file 50 will be described . the raw file 50 is created in a digital camera 60 when the digital camera 60 performs photographing , and can be stored in the card type memory 30 inserted in the digital camera 60 . the raw file 50 includes a header 52 for storing various information , raw data 54 to be unprocessed data in which an electric signal obtained from an electric charge generated by a light received by an image capturing unit ( for example , a ccd or a cmos ) of the digital camera 60 is converted into digital data , and thumbnail data 56 to be a reduced image obtained by developing the raw data 54 . the header 52 is a region having information about the numbers of longitudinal and horizontal pixels of the raw data 54 and a size of one pixel , for example . the raw data 54 have data on one of r , g and b per pixel . the case in which the image capturing unit is a color ccd will be taken as an example . in a color ccd 62 to be used for the digital camera 60 , generally , individual elements can identify separate colors from each other by covering a color filter for one of the three colors of r , g and b over the ccd . one r , one b and two gs are disposed in a window 62 a in two pixels by two pixels . for this reason , the raw data 54 have the data on one of r , g and b for each pixel . the raw data 54 usually have a high gradation representation of 8 to 16 bits for rgb and are subjected to a non - compression or a reversible compression in which the same state can be restored again even if a compression is performed . the thumbnail data 56 are obtained by developing the raw data 54 to have a predetermined pixel size ( for example , 160 × 120 ) in the digital camera 60 . therefore , color information about each pixel of the thumbnail data 56 depends on an image processing of the digital camera 60 performing photographing and an original region having an achromatic color is represented as the achromatic color . next , description will be given to an operation of the image processing apparatus 10 according to the embodiment which has such a structure , particularly , an operation to be performed in the execution of the jpeg file creating program . fig4 is a flowchart showing a jpeg file creating routine to be executed by the cpu 12 . the routine is stored in the program holder 28 a of the hdd 28 and is executed when a user gives a command for executing the jpeg file creating program from the input device 27 . when the routine is started , the cpu 12 first acquires the thumbnail data 56 and the raw data 54 from the raw file 50 stored in the card type memory 30 ( step s 100 ), and a pixel interpolation processing for the raw data 54 is performed ( step s 110 ). data on each pixel which are included in the raw data 54 are data on any one of r , g and b . therefore , a guess is given from colors of surrounding pixels per pixel , thereby complementing for colors other than the same pixels . since the processing of guessing a color is performed by connecting the same colors through a straight line or a curved line so as to be expressed , thus , it is referred to as a pixel interpolation processing . subsequently , the cpu 12 grasps a position of a pixel having an achromatic color of the thumbnail data 56 ( step s 120 ). more specifically , an absolute value (| r − g |) of a difference between r and g in values of r , g and b included in each pixel of the thumbnail data 56 and an absolute value (| b − g |) of a difference between b and g are calculated , and it is decided whether both of the absolute values are included in a predetermined small range ( which is smaller than three , for example ). when they are included in the small range , the pixel is set to have an achromatic color and a position of the pixel having the achromatic color is stored in the ram 16 . it is assumed that each of the values of r , g and b included in each pixel of the thumbnail data 56 is expressed in 256 gradations ( 8 bits ) of 0 to 255 corresponding to density . moreover , it is apparent that the values of r , g and b are almost equal to each other in the pixel having the achromatic color . in the case in which the respective differences between the values of r , g and b are included in the predetermined small range as described above , accordingly , it is assumed to decide that the pixel has the achromatic color as described above . when the absolute values of the differences between two sets of the values of r , g and b are included in the predetermined small range , similarly , the absolute value of the difference between a residual set of the values is necessarily included in a certain range . subsequently , the cpu 12 corresponds to a position of the pixel having the achromatic color of the thumbnail data 56 . a position of a pixel of the raw data 54 is calculated by a coordinate transformation ( step s 130 ). fig5 is a view for explaining the coordinate transformation . since the raw data 54 are digital data , they cannot be recognized as an image by human eyes . in fig5 , however , the raw data 54 are expressed in an image for convenience . an image of the thumbnail data 56 has a smaller size than the image of the raw data 54 . therefore , there is a plurality of pixels of the raw data 54 corresponding to the position of the pixel having the achromatic color of the thumbnail data 56 . in the embodiment , however , it is assumed that a central one of the pixels corresponds to the position of the pixel having the achromatic color of the thumbnail data 56 . then , σr , σg and σb are calculated by integrating the values of r , the values of g and the values of b respectively per pixel having the achromatic color of the raw data 54 which are obtained by the coordinate transformation ( step s 140 ), and white balance gains rc , gc and bc are calculated based on the values σr , σg and σb thus integrated ( step s 150 ). fig6 is an explanatory diagram related to the gains rc , gc and bc . at the step s 150 , the gain gc is set to have a value of 1 ( σg / σg ), the gain rc is set to be σg / σr and the gain bc is set to be σg / σb in such a manner that the values of r and b are coincident with the value of g on the basis of the value of g as shown in fig6 . then , the values of r , g and b of all the pixels of the raw data 54 are multiplied by the gains rc , gc and bc respectively , and the values obtained by the multiplication are set to be values of r , g and b obtained after the white balance correction processing ( step s 160 ). thereafter , there are performed a color correction processing of correcting a color corresponding to a human visual characteristic ( step s 170 ) and a gamma correction processing of correcting a change in a brightness for an input signal to be linear ( step s 180 ). thereafter , there is performed a jpeg compression for thinning color information with a low human sensitivity , dividing an image into small blocks and collecting colors , thereby reducing a file capacity ( step s 190 ), and a file name is given to a jpeg file obtained after the jpeg compression and the jpeg file is then stored in a predetermined region of the hdd ( step s 200 ), and the routine thus ends . according to the image processing apparatus 10 in accordance with the embodiment described above in detail , the white balance correction for the raw data 54 is performed by setting the thumbnail data 56 to be the developed data as an index . as compared with the case in which all of the rgb information about the respective pixels are read from the raw data 54 to perform the white balance correction , consequently , the correction processing can be performed more rapidly . in particular , the pixel having the achromatic color can be easily extracted from the thumbnail data 56 . by performing the white balance correction for the raw data 54 with the position of the pixel having the achromatic color set to be the index , therefore , it is possible to rapidly perform the correction processing . at this time , the pixel having the difference between the values of r , g and b included in a predetermined small range is set to be the pixel having the achromatic color in the thumbnail data 56 . therefore , it is possible to specify the position of the pixel having the achromatic color of the thumbnail data 56 with high precision . since a second embodiment is the same as the first embodiment except that a flowchart for a jpeg file creating routine is different , the same components have the same reference numerals and description thereof will be omitted . in the embodiment , a histogram representing a relationship between a gradation level and an appearance frequency ( the number of pixels ) in raw data 54 and a histogram representing a relationship between a gradation level and an appearance frequency in thumbnail data 56 are created for each of r , g and b , and the former shape is corrected to be coincident with the latter shape and a white balance correction for the raw data 54 is thus performed . fig7 shows an example of the flowchart illustrating the jpeg file creating routine employing the correction . when the routine is started , a cpu 12 first acquires the thumbnail data 56 and the raw data 54 from a raw file 50 stored in a card type memory 30 ( step s 300 ) and a pixel interpolation processing for the ram data 54 is performed ( step s 310 ). the pixel interpolation processing is the same as that in the step s 210 . subsequently , the cpu 12 creates a histogram for each of r , g and b of the thumbnail data 56 ( step s 320 ) and the same color correction processing ( step s 330 ) as that in the step s 170 is performed to the data obtained after the pixel interpolation processing for the raw data 54 , and a histogram for each channel of r , g and b of the data obtained after the color correction processing is then created ( step s 340 ). as shown in fig8 a and 8b , then , white balance gains rc , gc and bc are calculated in such a manner that a shape of the histogram for each channel of r , g and b created at the step s 340 is coincident with that of the histogram for each channel of r , g and b of the thumbnail data 56 ( step s 350 ). the shape of the histogram is corrected in such a manner that maximum gradation levels r_max , g_max and b_max of the histogram for each channel of r , g and b created at the step s 340 ( the maximum value of gradation levels with an appearance frequency of 1 or more ) is coincident with maximum gradation levels r_max , g_max and b_max of the histogram for each channel of r , g and b of the thumbnail data 56 . in that case , the white balance gains rc , gc and bc are calculated . subsequently , each channel for r , g and b of all pixels of the raw data 54 is multiplied by the gains rc , gc and bc respectively and the values obtained by the multiplication are set to be values for r , g and b obtained after the white balance correction processing ( step s 360 ). then , the same gamma correction processing ( step s 370 ) as the step s 180 and the same jpeg compression ( step s 380 ) as the step s 190 are performed , a file name is given to a jpeg file obtained after the jpeg compression and the jpeg file is stored in a predetermined region of an hdd ( step s 390 ), and the routine thus ends . according to the embodiment described above in detail , the white balance correction for the raw data 54 is performed by setting the thumbnail data 56 to be the developed data as an index . as compared with the case in which all of the rgb information about each pixel are read from the raw data 54 to perform the white balance correction , consequently , the correction processing can be performed more rapidly . in particular , it can be supposed that the thumbnail data 56 are subjected to the white balance correction for the raw data 54 by any processing method in a digital camera 60 . by simply adapting the shape of the histogram of the data obtained after performing the pixel interpolation and color correction to the raw data 54 to that of the histogram of the thumbnail data 56 , therefore , it is assumed that the white balance correction for the thumbnail data 56 is applied to the raw data 54 . consequently , it is possible to perform the white balance for the raw data 54 rapidly . it is apparent that the invention is not restricted to the embodiments but the invention can be executed in various modes without departing from the technical range thereof . while the white balance correction is performed after the execution of the pixel interpolation processing for the raw data 54 in the first and second embodiments , for example , it may be performed before the execution of the pixel interpolation processing . although the thumbnail data are utilized as the developed data in the first and second embodiments , it is possible to utilize , without a particular restriction , any data to be developed and stored simultaneously when storing the raw data on an image photographed by the digital camera 60 . for example , it is also possible to employ screen nail data as the developed data . here , the screen nail data are , for example , embedded image data for displaying . while it is not decided whether the pixel corresponding to the pixel having the achromatic color in the thumbnail data 56 of the raw data 54 is actually the pixel having the achromatic color or not in the first embodiment , it may be decided whether the pixel corresponding to the pixel having the achromatic color in the thumbnail data 56 of the raw data 54 is actually the pixel having the achromatic color or not and the white balance correction for the raw data 54 may be performed based on the value of each channel of r , g and b of the pixel . although the absolute value (| r − g |) of the difference between r and g and the absolute value ( b − g |) of the difference between b and g in the values of r , g and b included in each pixel of the thumbnail data 56 are calculated and it is decided whether both of the absolute values are included in the predetermined small range ( which is smaller than three , for example ) or not in order to grasp the position of the pixel having the achromatic color of the thumbnail data 56 at the s 120 in the first embodiment , a ratio ( r / g ) of r to g and a ratio ( b / g ) of b to g may be calculated to decide whether both of the ratios are included in a vicinal region of a value of “ 1 ” ( for example , 0 . 99 to 1 . 01 ) or not , and the pixel may be decided to have the achromatic color if they are included in the same region . while the raw data 54 are corrected in such a manner that the maximum gradation levels r_max , g_max and b_max of the histogram of the data obtained by performing the pixel interpolation and color correction over the raw data 54 are coincident with the maximum gradation levels r_max , g_max and b_max of the histogram in the thumbnail data 56 in the second embodiment , a mean value of the gradation level having an appearance frequency of 1 or more in the histogram may be used in place of the maximum gradation level or a value with which the appearance frequency of the histogram has a predetermined number ( for example , 50 % as a whole ), that is , a median may be used . in any case , it is possible to rapidly perform the white balance correction for the raw data 54 . in the second embodiment , it is also possible to compare a sum of the absolute values of the differences between both of the histograms ( the differences in the appearance frequency for the respective gradation levels ) obtained after the execution of the correction for causing the maximum gradation levels to be coincident with each other , a sum of the absolute values of the differences between both of the histograms obtained after the execution of the correction for causing the mean values to be coincident with each other and a sum of the absolute values of the differences between both of the histograms obtained after the execution of the correction for causing the medians to be coincident with each other , and to employ a correction processing in which the sum is a minimum as described above in order to correct the shape of the histogram of the data obtained after the execution of the pixel interpolation and color correction for the raw data 54 to be coincident with that of the histogram of the thumbnail data 56 . in the case in which the sum of the absolute values of the differences between both of the histograms obtained after the execution of each correction is the minimum , thus , it is possible to regard the shapes of both of the histograms to be the closest . therefore , it is preferable to employ the correction processing . the present disclosure relates to the subject matter contained in japanese patent application no . 2005 - 313056 ( filed on oct . 27 , 2005 ), which is expressly incorporated herein by reference in its entirely .