To correct aberration with a simple optical structure when light is measured.

In a spectrometer 1 which includes an input port 2 that inputs light from a light source, a spherical mirror 3 that reflects light input from the input port 2 and has a concave surface shape, a diffraction grating 4 that separates light reflected by the spherical mirror 3 and reflects the separated light to the spherical mirror 3, and has a spherical surface with an equal grating width and has a convex surface shape, a reflection optical system 5 that reflects backward light R obtained by reversing the forward light L to the spherical mirror 3 when the light reflected and separated by the spherical mirror 3 is forward light L, and an output port 6 that outputs the backward light R reflected by the spherical mirror 3 and the diffraction grating 4, the spectrometer makes the forward light L and the backward light R cancel aberration from each other and can correct the aberration, and can correct the aberration with a simple optical structure because of using a spherical mirror having a fixed curvature and a diffraction grating with an equal grating width.