PURPOSE: To reduce the occurrence of aberration by making the aperture angle of an image forming element of refractive index distribution type that forms an image forming optical system and the degree of superposition of images on a photosensitive face specified values.

CONSTITUTION: When the degree of superposition M on the light receiving face 3 is defined as X0/D where aperture angle θ of an image forming element of reflective index distribution type is larger than 9°, effective aperture of each optical element is D and radius of visual field is X0, the degree of superposition M=2.75+0.5x (where x is integer larger than 0) or thereabout. Adjusting device for distance between the face of original 2 and a photosensitive member 3 array 1 is provided according to the length of the length Z of the image forming element 7. The face of the original (the face of a body) is provided at a distance l0 at a position optically conjugate in relation to the photosensitive face (image forming face) 3 and image forming element 1 and made to unmagnification image forming relation.