Provided is a wavelength selective switch, which includes: an input/output unit; a dispersive portion; a condensing optical system; and the deflection portion. The input/output unit has input/output ports. The dispersive portion disperses signal light incident from the input/output ports. The condensing optical system condenses a plurality of signal light beams dispersed by the dispersive portion. The deflection portion has a plurality of deflection elements. The deflection elements deflect, along a second direction, the signal light beams condensed by the condensing optical system. In the condensing optical system, the aberration amount of the meridional component in a sagittal coma aberration remains substantially constant irrespective of an angle formed between the optical axis of the condensing optical system and a signal light beam incident on the condensing optical system from the input/output portion, at an incident position of the incident signal light beam at a certain height in the second direction.