In a wavelength selective switch, a holding member is used to rotate one end of optical fibers and a collimator array around a rotation axis to thereby change an incident angle of collimated light with respect to incident surfaces of a beam expander optical system. When the incident angle of the collimated light on the beam expander optical system is changed, an amount of variation in an emission angle of light from the beam expander optical system is not proportional (inversely proportional) to the magnification of the beam expander optical system. Thus, this wavelength selective switch can easily fine-tune the incident position (beam position) of light with respect to each reflecting surface of a MEMS mirror by rotating the holding member.