Provided is a wavelength selective switch, which includes: an input/output unit; a dispersive portion; a deflection portion; and an ovalization relay optical system. In the input/output unit, input/output portions are two-dimensionally arranged. The dispersive portion is capable of dispersing signal light along a first plane. The deflection portion deflects the signal light. The ovalization relay optical system condenses the signal light beams on to a first conjugate point. The ovalization relay optical system makes a beam waist forming position along a first direction coincide with the first conjugate point. The ovalization relay optical system condenses signal light, in a second direction, onto a first condensing point. The ovalization relay optical system makes the first condensing point conjugate to the first conjugate point. The ovalization relay optical system ovalizes the beam shape of the signal light beams incident on the deflection element.