now referring to the drawings , preferred embodiments of the invention are described below . fig2 is an external perspective view showing a first embodiment of the invention , fig3 a to 3d are front , plan , rear and side views respectively showing the embodiment , and fig4 is a view showing the internal structure of the embodiment . a dental x - ray image detecting apparatus 11 comprises an x - ray detecting element 22 , an apparatus case 23 for housing the element , a cable 24 , and a buffer amplifier 27 . referring to fig4 the x - ray detecting element 22 is illustrated below . the x - ray detecting element 22 is configured by stacking , on a substrate 31 , a ccd ( charge coupled device ) 32 , an optical fiber plate 33 , and a scintillator ( fluorescent material ) 34 in this sequence . an x - ray irradiated on an x - ray irradiation face 41 passes through the apparatus case 23 that is in a substantially rectangular parallelepiped shape like a matchbox thereby reaching the scintillator 34 . the scintillator 34 converts the x - ray having reached the scintillator 34 into a visible light . the optical fiber plate 33 is configured by bundling optical fibers into a plate - like shape , and guides the visible light from the scintillator 34 to the ccd 32 . the visible light from the optical fiber plate 33 is received and converted into an electric signal by the ccd 32 . the apparatus case 23 has a size which allows the apparatus case to be inserted into the mouth of the subject , and includes the x - ray irradiation face 41 on which an x - ray irradiates . both the x - ray irradiation face 41 and a back face 42 which is on the opposite side of the irradiation face are formed to be substantially rectangular . a projected portion 43 is formed nearby one of the longer sides of the back face 42 . the projected portion 43 is positioned in the vicinity of the center of the area nearby the longer side of the back face 42 . in the projected portion 43 , the cable 24 is connected , through a side face 44 adjacent to the back face 42 of the apparatus case 23 , to the x - ray detecting element in the apparatus case . the x - ray detecting element 22 is incorporated in the region of the apparatus case 23 other than the projected portion 43 . the apparatus case 23 has a size of , for example , 36 mm × 26 mm × 9 mm . as described above , the cable 24 is connected to the projected portion 43 of the apparatus case 23 , and drawn out from the apparatus case 23 in a direction z . the direction z is parallel with the x - ray irradiation face 41 and perpendicular to the longitudinal direction y of the apparatus case 23 . in the apparatus case 23 , the cable 24 is electrically connected to the ccd 32 , and the electric signal from the ccd 32 is transmitted through the cable 24 to the outside of the apparatus case 23 . the provision of the projected portion 43 ensures a space for electrically connecting . the buffer amplifier 27 is disposed in a position which is on the cable 24 separated from the apparatus case 23 by about 10 cm . for the sake of simplicity , the buffer amplifier 27 is not shown in the drawings other than fig2 . the dental x - ray image detecting apparatus 11 configured as shown above is used in a manner of that the apparatus case 23 , the cable 24 , and the buffer amplifier 27 are covered by a disposable cover 29 . the use of the disposable cover 29 prevents entry of water into the apparatus and infection in the mouth . the cable 24 may be drawn out in an oblique direction instead of a direction parallel to the x - ray irradiation face 41 . fig5 and 6 are perspective views showing a second embodiment of the invention . a dental x - ray image detecting apparatus 12 is a partial modification of the dental x - ray image detecting apparatus 11 of fig2 . a flat convex connector 51 is formed at the tip end of the cable 24 , and a concave connector 52 of the x - ray detecting element is formed on the side face 44 of the projected portion 43 of the apparatus case 23 . the flat convex connector 51 includes contacts which are made of an electrically conductive material , and lock levers 55 at both sides thereof . the lock levers 55 are made of an elastic material of resin or metal so as to have flexibility . the concave connector 52 has a shape which substantially corresponds to that of the flat convex connector 51 , and includes contacts which are made of an electrically conductive material in the same manner as the flat convex connector 51 . the flat convex connector 51 is inserted into the concave connector 52 to be attached thereto . in the course of the attachment , the contacts of the connectors are contacted with each other thereby establishing an electrical connection between the connectors . the contacts of the flat convex connector 51 are electrically connected to the conductors inside the cable 24 , and those of the concave connector 52 are electrically connected to the x - ray detecting element 22 , with the result that the cable 24 is electrically connected to the x - ray detecting element 22 . the inserted flat convex connector 51 is fixed to the concave connector 52 by the lock levers 55 . when the lock levers 55 are pressed from both sides , the inserted flat convex connector 51 can be detached from the concave connector 52 . a dental x - ray image detecting apparatus 13 of fig6 is a partial modification of the dental x - ray image detecting apparatus 12 of fig5 . the flat convex connector 51 is replaced with a cylindrical convex connector 53 , and the concave connector 52 to a cylindrical concave connector 54 . the cylindrical convex connector 53 , which is circular in cross - section , includes a plurality of pins at the tip end , and a rubber ring 56 for watertight sealing . the cylindrical concave connector 54 includes insertion holes into which the pins of the cylindrical convex connector 53 are respectively inserted , and has a shape which substantially corresponds to that of the cylindrical convex connector 53 . when the cylindrical convex connector 53 is inserted into the cylindrical concave connector 54 , the pins are inserted into the respective insertion holes , thereby establishing an electrical connection between the connectors . during a period when the cylindrical convex connector 53 is inserted into the cylindrical concave connector 54 , the connectors are fixed to each other so as to be hardly separated from each other , by the rubber ring 56 . the fixation of the cylindrical convex connector 53 and the cylindrical concave connector 54 is not restricted to that based on the rubber ring 56 , and may be realized by thread coupling or a fastening structure using a cap nut . fig7 a and 8b are perspective views showing a third embodiment of the invention . a dental x - ray image detecting apparatus 14 is a partial modification of the dental x - ray image detecting apparatus 12 of fig5 . the projected portion 43 shown in fig5 is removed away , the concave connector 52 is replaced with a concave connector 64 , and the flat convex connector 51 is replaced with connectors of two kinds , i . e ., first and second convex connectors 61 and 62 . in accordance with these changes , the cable 24 is replaced with a first cable 25 connected to the first convex connector 61 , and a second cable 26 connected to the second convex connector 62 . the concave connector 64 is formed on a position where the projected portion 43 is formed on the back face 42 of the apparatus case 23 , in place thereof , and includes contacts 65 of the x - ray detecting element which are made of an electrically conductive material . both the first and second convex connectors 61 and 62 are shaped into such a form that the convex connectors are slidably attached to the concave connector 64 . spring contacts 63 are disposed in portions where the convex connectors contact with the contacts 65 of the concave connector 64 . either one of the first and second convex connectors 61 and 62 is attached to the concave connector 64 . as shown in fig8 a , when the first convex connector 61 is attached to the concave connector 64 , the first cable 25 is drawn out in the direction z perpendicular to the longitudinal direction of the apparatus case 23 . as shown in fig8 b , when the second convex connector 62 is attached to the concave connector 64 , the second cable 26 is drawn out in the direction y which is parallel with the longitudinal direction of the apparatus case 23 . fig9 is a perspective view showing a fourth embodiment of the invention . a dental x - ray image detecting apparatus 15 is a partial modification of the dental x - ray image detecting apparatus 12 of fig5 . the flat convex connector 51 is replaced with a convex connector 71 , and the concave connector 52 is replaced with a concave connector 72 . the convex connector 71 and the concave connector 72 are used for constituting a pga ( pin grid array ). the respective faces of the connectors which are opposed to each other are formed to be square . a plurality of pins on the convex connector 71 are arranged so as to be rotationally symmetric by 90 degrees . pin insertion holes on the concave connector 72 are also arranged in the same manner as the above . the concave connector 72 itself has a shape which substantially corresponds to that of the convex connector 71 . since the convex connector 71 can be attached to the concave connector 72 in one of four directions , the cable 24 can be drawn out in any one of the four directions . alternatively , the attaching directions may be restricted to two directions which are mechanically perpendicular to each other . in the alternative , also the draw - out directions of the cable 24 are restricted to the two directions . fig1 is a perspective view showing a fifth embodiment of the invention , fig1 is a section view of the embodiment , and fig1 is an exploded perspective view of the embodiment . a dental x - ray image detecting apparatus 16 is a partial modification of the dental x - ray image detecting apparatus 15 of fig9 . the convex connector 71 is replaced with a convex connector 81 , and the concave connector 72 is replaced with a concave connector 82 . the convex connector 81 is attached to the concave connector 82 of the x - ray detecting element , via a fixing ring 83 . in the convex connector 81 , a surface which is contacted with the concave connector 82 has a circular shape , and spring contacts 84 are formed thereon . projections 831 of the fixing ring 83 are respectively fitted into cutaway portions 821 of the concave connector 82 and rotated , whereby the convex connector 81 is rotatively connected to the concave connector 82 . also in the concave connector 82 , a surface which is contacted with the convex connector 81 has a circular shape , and concentric sliding contacts 85 are formed thereon . when the convex connector 81 is rotated with respect to the concave connector 82 , the spring contacts 84 are kept to be contacted with the sliding contacts 85 , during the rotation . the spring contacts 84 are electrically connected to the cable 24 via through holes 87 which are formed in an electrically insulating substrate 86 of the convex connector 81 . on the other hand , the sliding contacts 85 are electrically connected to the ccd 32 via through holes 88 which are formed on the substrate 31 . fig1 a to 16b are views showing modifications of the fifth embodiment . when disposing the spring contacts 84 , the number of sets thereof is not restricted to two . as shown in fig1 a and 13b , one set may be disposed , or three sets which are rotationally symmetric by 120 degrees may be disposed . furthermore , the arrangement may be performed in any way , and four or more sets may be disposed . the shape of contacts is not restricted to that mentioned above . as shown in fig1 a to 14c , each contact may be a plate spring , a dome - shaped spring , or electrically conductive rubber . the sliding contacts 85 are not required to be concentric circles which form one complete rotation . when the rotation angle of the convex connector 81 is mechanically limited , or as shown in fig1 a and 13b , the number of sets of the spring contacts 84 is changed , each sliding contact can be configured to rotate by three quarters or one quarter of one complete rotation as shown in fig1 a to 15c . in these cases , it is not required to route wiring to the back face of the substrate via through holes , and patterns can be routed along the substrate surface from areas where the circles of the sliding contacts are not formed . as shown in fig1 a and 16b , the rotation angle of the convex connector 81 may be limited to 90 degrees or 180 degrees . fig1 is a perspective view showing a sixth embodiment of the invention , and fig1 a and 18b are views showing the patterns of sliding contacts of the embodiment . a dental x - ray image detecting apparatus 17 is a partial modification of the dental x - ray image detecting apparatus 16 of fig1 . as shown in fig1 a , the pattern of sliding contacts 94 of the dental x - ray image detecting apparatus 17 is configured by connecting parallel lines to portions in which a rotation of one - quarter of a concentric circle is conducted . a window 91 having an arcuate portion 92 and a linear portion 93 is formed on the back face 42 of the apparatus case 23 , in accordance with the pattern of the sliding contacts 94 . when the spring contacts of the convex connector are contacted with the concentric portion 95 of the sliding contacts 94 , the cable 24 is drawn out through the arcuate portion 92 of the window 91 . when the spring contacts are contacted with parallel - line portion 96 of the sliding contacts 94 , the cable 24 is drawn out through the linear portion 93 of the window 91 . in the arcuate portion 92 , the draw - out direction of the cable 24 can be rotated by 90 degrees , and , in the linear portion 93 , the draw - out position of the cable 24 can be moved in the direction y . as shown in fig1 b , in the pattern of the sliding contacts 94 , the concentric portion 95 may be configured so that a rotation of three - quarter of a concentric circle is conducted . in this case , the arcuate portion 92 is widened to allow the draw - out direction of the cable 24 to be rotated by 270 degrees . alternatively , the pattern of the sliding contacts 94 may be configured only by the parallel - line portion 96 . in the alternative , the draw - out direction of the cable 24 is fixed to the direction z . the moving direction and distance of the draw - out position of the cable 24 are not restricted . fig1 to 21 are sets of three views respectively showing adaptors 101 to 103 for the dental x - ray image detecting apparatuses 11 to 17 . the adaptor 101 of fig1 is made of a material having elasticity and softness , such as silicone rubber , polypropylene , or natural rubber , and has a shape which substantially corresponds to the outer shape of the apparatus case 23 , so as to cover the x - ray irradiation face 41 and the side faces of the case . in the same manner as the x - ray irradiation face 41 , an x - ray irradiation face 111 of the adaptor 101 which corresponds to the x - ray irradiation face 41 is flat . in the same manner as the adaptor 101 , the adaptor 102 of fig2 is made of a soft and elastic synthetic resin and has a shape which substantially corresponds to the outer shape of the apparatus case 23 , so as to cover the x - ray irradiation face 41 and the side faces of the case . an x - ray irradiation face 112 of the adaptor 102 is curved so that the center portion is swollen in both the directions y and z , thereby allowing the face to be fitted to the inside of the dentition . in the same manner as the adaptor 101 , the adaptor 103 of fig2 is made of a soft and elastic synthetic resin and has a shape which substantially corresponds to the outer shape of the apparatus case 23 , so as to cover the x - ray irradiation face 41 and the side faces of the case . in the same manner as the x - ray irradiation face 112 of the adaptor 102 , an x - ray irradiation face 113 of the adaptor 103 is curved so that the center portion is swelled in both directions y and z , thereby allowing the face to be fitted to the inside of the dentition . the adaptor 103 has engaging portions 114 which cover the outer edge portion of the back face 42 of the apparatus case 23 . the engagement of the apparatus case 23 is further enhanced by the engaging portions 114 . fig2 is a perspective view showing an adaptor 104 for the dental x - ray image detecting apparatuses 11 to 17 . the adaptor 104 of fig2 is made of a soft and elastic synthetic resin and covers only the side faces of the apparatus case 23 in the same manner as the adaptor 101 . although the ccd 32 is used for the x - ray detecting element 22 , the element may be an imaging element of another kind such as an mos image sensor . the apparatus case 23 may be integrated with the cable 24 so as to be detachably attached to the x - ray detecting element . also a configuration in which the apparatus case 23 is provided with an electric connector which is electrically connected to the x - ray detecting element and the cable is connected to the electric connector is in the scope of the invention . the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof . the present embodiments are therefore to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein .