A spectrophotometer in which a normal plane to a diffraction grating is inclined with respect to an optical axis of an incident light passing through a slit, the normal plane to the diffraction grating passing through an intersection point between the optical axis of the incident light i and a grating surface of the diffraction grating. The diffraction grating and a photodiode array PDA are placed such that the photodiode array PDA is parallel to the normal plane to the diffraction grating and that a normal plane to the photodiode array PDA includes a line that is symmetrical to the optical axis of the incident light i about the normal plane to the diffraction grating.