To provide a detection optical system having a spectrum detection function, capable of enhancing the amount of light detected by improving diffraction efficiency.

A detection optical system 10 is adopted which includes: a transmission type VPH diffraction grating 11 which divides fluorescent light from a sample into a plurality of wavelength bands; a rotation mechanism which rotates the VPH diffraction grating 11 around an axis line L orthogonal to an incident optical axis of the fluorescent light from the sample and an emission optical axis from the VPH diffraction grating 11; a light detection part 15 which detects the fluorescent light divided by the VPH diffraction grating 11 and from the sample; and correction means which corrects an incident position to the light detection part 15 in accordance with displacement of an optical axis caused by rotation of the VPH diffraction grating 11 in synchronization with the rotation mechanism.