in fig1 and 2 , a test piece 1 in accordance with a first embodiment of the present invention comprises a substrate 2 which is a slide glass in this particular embodiment , and a plurality of different cdnas disposed in a plurality of positions on opposite sides ( upper and lower sides ) of the substrate 2 . the base sequences of the cdnas are known and correspond to different dnas . the kind of each cdna and the position of each cdna are predetermined . as shown in fig2 the cdnas on the upper side of the substrate 2 and those on the lower side of the substrate 2 are positioned not to overlap each other in the direction of thickness of the substrate 2 . further the upper and lower sides of the substrate 2 have been subjected to surface treatment so that the cdnas are bonded to the surfaces and accordingly , the cdnas on the lower side of the substrate 2 cannot peel off the substrate 2 . the thickness of the substrate 2 is about 1 mm , and each of the cdnas is disposed on the surface of the substrate 2 in a spot of a diameter 30 to 100 μm with the spot intervals of about 300 μm . fig3 shows a test piece in accordance with a second embodiment of the present invention . the test piece shown in fig3 is provided with a substrate formed of a pair of substrate segments 2 a and 2 b which are bonded together with a spacer 3 interposed therebetween . the cdnas are disposed on the upper surfaces of the respective substrate segments 2 a and 2 b not to overlap each other in the direction of thickness of the substrate segments 2 a and 2 b . the spacer 3 may either be discontinuous or continuous over the entire periphery of the substrate segments 2 a and 2 b . fig4 a and 4b show an image information read - out system in accordance with a third embodiment of the present convention for reading out image information from the test piece 1 shown in fig1 . the image information read - out system comprises a transparent sample table 20 on which the test piece 1 , having applied with an organism - originating substance labeled with a fluorescent dye , is supported at its four corners , a laser 30 which emits a laser beam l in a wavelength band exciting the fluorescent dye , a lens 31 which converges the laser beam l as emitted from the laser 30 into a thin laser beam , a photomultiplier 40 which photoelectrically detects fluorescence k 1 and k 2 emitted from the cdnas upon exposure to the laser beam l ( k 1 represents the fluorescence emitted from the cdnas on the upper surface of the substrate 2 and k 2 represents the fluorescence emitted from the cdnas on the lower surface of the substrate 2 ), an optical head 50 which causes the laser beam l to impinge upon the test piece 1 on the sample table 20 and leads the fluorescence k 1 or k 2 to the photomultiplier 40 , a laser beam cut filter 41 disposed on the optical path between the optical head 50 and the photomultiplier 40 , a condenser lens 55 which is disposed between the test piece 1 and the photomultiplier 40 and forms a confocal optical system together with a lens 53 , an aperture plate 56 which has an aperture 56 a which permits to impinge upon the lens 41 only light from a portion of the test piece 1 on which the laser beam l is converged by the lens 53 , a main scanning system 60 which moves the optical head 50 in the direction of arrow x at a constant speed , a sub - scanning means 80 which moves the laser 30 , the optical head 50 , the condenser lens 55 , the aperture plate 56 , the laser beam cut filter 41 and the photomultiplier 40 in the direction of arrow y ( perpendicular to the direction of arrow x ) integrally with each other , a logarithmic amplifier 42 which logarithmically amplifies a detecting signal output from the photomultiplier 40 , and an a / d converter 43 which digitizes the amplified detecting signal . the laser 30 is arranged to emits the laser beam l in the direction of arrow x , and the photomultiplier 40 is arranged to detect the fluorescence k 1 or k 2 impinging thereupon in the direction of arrow x . the optical head 50 comprises a plane mirror 51 which reflects the thin laser beam l , traveling in the direction of arrow x , in a direction perpendicular to the surfaces of the test piece 1 , a mirror 52 which is provided with an aperture 52 a through which the laser beam l reflected by the plane mirror 51 impinges upon the test piece 1 and reflects the major parts of the fluorescence k 1 or k 2 , emitted downward from the lower surface of the test piece 1 , to impinge upon the photomultiplier 40 , and the lens 53 which collimates the fluorescence k 1 or k 2 which emits downward from the test piece 1 as divergent light . the plane mirror 51 , the mirror 52 with the aperture 52 a and the lens 53 are integrated into a unit . the lens 53 is movable in the direction of thickness of the test piece 1 or in the direction of arrow z to move a focal point of the lens 53 selectively to the upper surface of the substrate 2 and to the lower surface of the same . when the fluorescence k 1 from the upper surface of the test piece 1 is to be detected , the focal point of the lens 53 is moved to the upper surface of the test piece 1 and when the fluorescence k 2 from the lower surface of the test piece 1 is to be detected , the focal point of the lens 53 is moved to the lower surface of the test piece 1 , whereby the florescence k 1 and k 2 can be collimated to beams of substantially the same diameters . the laser beam cut filter 41 is a filter which transmits the fluorescence k 1 and k 2 but does not transmit the laser beam l so that a part of the laser beam l scattered by the test piece 1 , the sample table 20 and the like cannot impinge upon the photomultiplier 40 . operation of the image information read - out system of this embodiment will be described , hereinbelow . the position of the lens 53 is first adjusted so that the focal point of the lens 53 is on the upper surface of the substrate 2 . then the main scanning means 60 moves the optical head 50 at a constant high speed in the direction of arrow x . at each moment during movement of the optical head 50 , the laser 30 emits a laser beam l in the direction of arrow x and the lens 31 converges the laser beam l into a thin laser beam . the thin laser beam l enters the optical head 50 . the laser beam l is then reflected upward by the plane mirror 51 and impinges upon a fine area on the upper surface of the test piece 1 through the aperture 52 a of the mirror 52 and the lens 53 . when an organism - originating substance labeled with fluorescent dye exists in the fine area exposed to the laser beam l , the fluorescent dye is excited by the laser beam l and emits fluorescence k 1 . the fluorescence k 1 spread around the area and the part of the fluorescence k 1 traveling downward from the lower surface of the test piece 1 is collimated by the lens 53 of the optical head 50 into a substantially parallel downward beam and impinges upon the mirror 52 . though the part of the fluorescence k 1 impinges upon the aperture 52 a travels further downward through the aperture 52 a ( the diameter of the aperture 52 a is sufficiently small as compared with the beam diameter ), the major part of the fluorescence k 1 is reflected by the mirror 52 to travel in the direction of arrow x and to impinge upon the photomultiplier 40 through the condenser lens 55 , the aperture 56 a of the aperture plate 56 and the laser bean cut filter 41 . though a part of the laser beam l impinging upon the test piece 1 is scattered by the test piece 1 , the sample table 20 and the like and travels toward the photomultiplier 40 , it is prevented from impinging upon the photomultiplier 40 by the laser beam cut filter 41 . further since the test piece 1 and the photomultiplier 40 are optically connected by a confocal optical system , fluorescence from a part of the test piece other than the part exposed to the laser beam l is prevented from impinging upon the photomultiplier 40 and blur of a fluoresce image obtained can be avoided even if the area exposed to the laser beam l is shifted or enlarged . the fluorescence k 1 impinging upon the photomultiplier 40 is photoelectrically detected by the photomultiplier 40 and read out as an electric signal . the electric signal is amplified by the amplifier 42 and is converted to a digital signal by the a / d converter 43 . during these steps , the optical head 50 is kept moved in the direction of arrow x by the main scanning system 60 , and a digital signal is output from the a / d converter 43 for each main scanning position on the test piece 1 . each time the main scanning along one line is ended , the sub - scanning means 80 slightly moves the laser 30 , the optical head 50 , the laser beam cut filter 41 and the photomultiplier 40 in the direction of arrow y ( sub - scanning ) and the main scanning is repeated . the sub - scanning may be effected in parallel to the main scanning . thus the entire area of the upper surface of the test piece 1 is two - dimensionally scanned by the laser beam l , and image information representing the distribution of the organism - originating substances labeled by the fluorescent dye on the upper surface of the substrate 2 is obtained . thereafter the optical head 50 is returned to the initial position by the main scanning means 60 and the sub - scanning means 80 . then the lens 53 is moved downward by d 0 ( fig5 ) so that the focal point of the lens 53 is on the lower surface of the substrate 2 . then the fluorescence k 2 emitted from the lower surface of the test piece 1 is detected and converted to a digital signal in the same manner as described above and image information representing the distribution of the organism - originating substances labeled by the fluorescent dye on the lower surface of the substrate 2 is obtained . the image information representing the distribution of the organism - originating substances labeled by the fluorescent dye on the upper and lower surfaces of the substrate 2 is displayed on a monitor ( not shown ). thus in the image information read - out system of this embodiment , image information can be read out from opposite sides of the test piece 1 of the first embodiment of the present invention , where cdnas are disposed on opposite sides of the substrate 2 . the image information read - out system of this embodiment can be used to read out image information from the test piece of the second embodiment of the present invention shown in fig3 . in this case , the lens 53 is moved so that the focal point of the lens 53 is selectively moved to the upper surface of the substrate segment 2 a and that of the substrate segment 2 b . though , in the embodiment described above , the fluorescence k 1 emitted from the cdnas on the upper surface of the substrate 2 and the fluorescence k 2 emitted from the cdnas on the lower surface of the substrate 2 are separately detected by moving the focal point of the lens 53 forming a confocal optical system , it is possible to separately detect the fluorescence k 1 and the fluorescence k 2 by moving the aperture plate 56 along the optical axis with the lens 53 kept stationary . that is , when the laser beam l is projected onto the test piece 1 with the focal point of the lens 53 set at the middle between the upper and lower surfaces of the substrate 2 , the fluorescence k 1 is emitted from the cdnas on the upper surface of the substrate 2 and the fluorescence k 2 is emitted from the cdnas on the lower surface of the substrate 2 . depending on the position of the aperture plate 56 along the optical axis , only one of the fluorescence k 1 and the fluorescence k 2 can pass through the aperture 56 a in the aperture plate 56 . fig6 a and 6b show an image information read - out system in accordance with a fourth embodiment of the present invention for reading out image information from the test piece 1 shown in fig1 . the image information read - out system of this embodiment comprises a sample table 20 , a first laser 30 a , a first lens 31 a , a first photomultiplier 40 a , an optical head 50 , a first laser beam cut filter 41 a , a first condenser lens 55 a , a first aperture plate 56 a , a main scanning system 60 , a sub - scanning means 80 , a first logarithmic amplifier 42 a , and a first a / d converter 43 a , which are basically the same as the sample table 20 , the laser 30 , the lens 31 , the photomultiplier 40 , the optical head 50 , the laser beam cut filter 41 , the condenser lens 55 , the aperture plate 56 , the main scanning system 60 , the sub - scanning means 80 , the logarithmic amplifier 42 , and the a / d converter 43 employed in the third embodiment . the image information read - out system of this embodiment further comprises a second laser 30 b , a second lens 31 b , a second condenser lens 55 b , a second aperture plate 56 b , a second laser beam cut filter 41 b , a second photomultiplier 40 b , a second logarithmic amplifier 42 b , a second a / d converter 43 b , a polarization beam splitter 62 which transmits the laser beam l 1 emitted from the first laser 30 a and reflects the laser beam l 2 emitted from the second laser 30 b , and a half - silvered mirror 63 which transmits a part of the fluorescence k 1 and the fluorescence k 2 to impinge upon the first photomultiplier 40 a , and reflects the other part of the fluorescence k 1 and the fluorescence k 2 to impinge upon the second photomultiplier 40 b . the first and second lenses 31 a and 31 b are identical to each other . the first and second lasers 30 a and 30 b are basically identical to each other except that the laser beam li emitted from the first laser 30 a is polarized in the vertical direction as seen in fig6 b and the laser beam l 2 emitted from the second laser 30 b is polarized in a direction perpendicular to the surface of the paper on which fig6 b is drawn . with this arrangement , the laser beam l 1 transmits the polarization beam splitter 62 and the laser beam l 2 is reflected by the same . the distance d 2 between the beam radiating end of the 16 second laser 30 b and the second lens 31 b is set larger than the distance d 1 between the beam radiating end of the first laser 30 a and the first lens 31 a so that the diameter of the laser beam l 1 on the upper surface of the substrate 2 becomes equal to the diameter of the laser beam l 2 on the lower surface of the substrate 2 . further , the distance d 2 between the second condenser lens 55 b and the second aperture plate 56 b is set larger than the distance d 1 between the first condenser lens 55 a and the first aperture plate 56 a . in the fourth embodiment , the leaser beams l 1 and l 2 are simultaneously emitted from the first and second lasers 30 a and 30 b , and the fluorescence k 1 and the fluorescence k 2 emitted respectively from the upper and lower surfaces of the test piece 1 are simultaneously detected . the fluorescence k 1 and the fluorescence k 2 emitted respectively from the upper and lower surfaces of the test piece 1 simultaneously travel in the direction of arrow x . the fluorescence k 1 and the fluorescence k 2 emitted respectively from the upper and lower surfaces of the test piece 1 are separated by the half - silvered mirror 63 to parts which respectively travel to the first and second photomultipliers 40 a and 401 b . either of the parts includes both the fluorescence k 1 and the fluorescence k 2 , and accordingly , the distance d 2 between the second condenser lens 55 b and the second aperture plate 56 b is set larger than the distance d 1 between the first condenser lens 55 a and the first aperture plate 56 so that only the fluorescence k 1 can pass through the aperture of the first aperture plate 56 a and only the fluorescence k 2 can pass through the aperture of the second aperture plate 56 b , whereby the fluorescence k 1 and the fluorescence k 2 are separately detected by the photomultipliers 40 a and 40 b , respectively . operation of the image information read - out system of this embodiment will be described , hereinbelow . when the laser beams l 1 and l 2 are projected onto the upper and lower surfaces of the test piece 1 , fluorescence k 1 and fluorescence k 2 are emitted from the upper and lower surfaces of the test piece 1 , respectively , and simultaneously travel in the direction of arrow x as a light bundle . the light bundle is divided into two light bundles by the half - silvered mirror 63 , one traveling toward the first photomultiplier 40 a and the other traveling toward the second photomultiplier 40 b . the fluorescence k 1 included in said one light bundle impinges upon the first photomultiplier 40 a through the first condenser lens 55 a , the first aperture plate 56 a and the first laser beam cut filter 41 a and is detected by the first photomultiplier 40 a , whereas the fluorescence k 2 included in said the other light bundle impinges upon the second photomultiplier 40 b through the second condenser lens 55 b , the second aperture plate 56 b and the second laser beam cut filter 41 b and is detected by the second photomultiplier 40 b . the fluorescence k 1 and the fluorescence k 2 are photoelectrically converted to electric signals respectively by the first and second photomultipliers 40 a and 40 b , and the electric signals are amplified by the first and second amplifiers 42 a and 42 b and then digitized by the first and second a / d converters 43 a and 43 b . then visible images are displayed on a monitor ( not shown ) on the basis of the digitized electric signals . thus also in the image information read - out system of this embodiment , image information can be read out from opposite sides of the test piece 1 of the first embodiment of the present invention , where cdnas are disposed on opposite sides of the substrate 2 . the image information read - out system of this embodiment can be used to read out image information from the test piece of the second embodiment of the present invention shown in fig3 . in this case , positions of the first and second lenses 31 a and 31 b and the first and second aperture plates 56 a and 56 b are adjusted to read out image information from the substrate segments 2 a and 2 b . though , in the embodiments described above , the cdnas are disposed not to overlap each other in the direction of thickness of the substrate 2 or the substrate segments 2 a and 2 b , they may overlap each other in the direction of thickness of the substrate 2 or the substrate segments 2 a and 2 b when the image information read - out system comprises a confocal optical system .