A detection optical system (10; 20; 30) comprising: a transmissive volume phase holographic, VPH, diffraction grating that disperses light from a specimen into a plurality of wavelength bands; a rotating mechanism that rotates the VPH diffraction grating about an axial line that is perpendicular to an incident optical axis of the light from the specimen and an emission optical axis from the VPH diffraction grating; and a light detection portion (15; 17) that detects the light from the specimen that has been dispersed by the VPH diffraction grating; characterized by a correcting portion (13; 19) that corrects, in synchronization with the rotating mechanism, an incident position on the light detection portion (15; 17) for light from the specimen in accordance with a displacement of the emission optical axis caused by the rotation of the VPH diffraction grating, wherein the correction portion corrects the displacement angle Δα which is the difference between - a first emission angle α 1, at which the light from the specimen having a predetermined wavelength (λ 600) is emitted from the VPH diffraction grating when the VPH diffraction grating is at a first rotation angle θ 1 and - a second emission angle α 2, at which the light from the specimen having the predetermined wavelength (λ 600) is emitted from the VPH diffraction grating when the VPH diffraction grating is at a second rotation angle θ 2. a transmissive volume phase holographic, VPH, diffraction grating that disperses light from a specimen into a plurality of wavelength bands; a rotating mechanism that rotates the VPH diffraction grating about an axial line that is perpendicular to an incident optical axis of the light from the specimen and an emission optical axis from the VPH diffraction grating; and a light detection portion (15; 17) that detects the light from the specimen that has been dispersed by the VPH diffraction grating; characterized by a correcting portion (13; 19) that corrects, in synchronization with the rotating mechanism, an incident position on the light detection portion (15; 17) for light from the specimen in accordance with a displacement of the emission optical axis caused by the rotation of the VPH diffraction grating, wherein the correction portion corrects the displacement angle Δα which is the difference between - a first emission angle α 1, at which the light from the specimen having a predetermined wavelength (λ 600) is emitted from the VPH diffraction grating when the VPH diffraction grating is at a first rotation angle θ 1 and - a second emission angle α 2, at which the light from the specimen having the predetermined wavelength (λ 600) is emitted from the VPH diffraction grating when the VPH diffraction grating is at a second rotation angle θ 2. - a first emission angle α 1, at which the light from the specimen having a predetermined wavelength (λ 600) is emitted from the VPH diffraction grating when the VPH diffraction grating is at a first rotation angle θ 1 and - a second emission angle α 2, at which the light from the specimen having the predetermined wavelength (λ 600) is emitted from the VPH diffraction grating when the VPH diffraction grating is at a second rotation angle θ 2.