To provide a variable wavelength filter in which an arbitrary wavelength selection characteristic is easily available.

Light is made incident on a diffraction grating 13 and output with a distribution of angles different for every wavelength. The distributed light is turned into a belt-shaped light with a lens 14, and made incident on a mirror substrate 15 having selective reflection regions 17-1 to 17-x. By moving the mirror substrate 15 in a direction different from the distribution direction of the belt-shaped light, the light having either one wavelength is reflected. And the light returned back to the diffraction grating 13 is reflected to the original incident direction of light. Thus, the variable wavelength filter, in which the light having an arbitrary wavelength is selected by moving the mirror substrate 15, is achieved.