A wavelength selective switch includes a dispersion optical system dispersing wavelength multiplexing light obtained by multiplexing the plurality of frequency components to the plurality of frequency components by giving a dispersion angle having nonlinear frequency dependency to each of a plurality of frequency components; a light deflection element deflecting the plurality of frequency components; a condensing element condensing the plurality of frequency components on the light deflection element; and a prism optical system optically coupled to the dispersion optical system and the condensing element, and adapting spatial positions of the frequency components incident on the light deflection element to change linearly for frequencies by linearizing the frequency dependency of the dispersion angles and making incident the plurality of frequency components on the condensing element.