A wavelength selection switch includes a diffraction grating that spectrally separates entering light, a focusing optical system that receives the light spectrally separated by the diffraction grating and focuses the light at a predetermined position, a deflection member that is arranged at the predetermined position and includes deflection elements that deflect the light entering through the focusing optical system, a base on which the diffraction grating, the focusing optical system, and the deflection member are mounted, and a deflection member position adjusting member that adjusts a position of the deflection member with respect to one of the diffraction grating and the focusing optical system. The deflection member position adjustment member includes a positioning member that determines a reference position of the deflection member in a direction of spectral separation of the diffraction grating, and a guide member that guides the deflection member in the direction of spectral separation.