An optical switch switches ports through which a beam is input and output and includes an optical system through which the beam passes; a movable reflector that is enclosed in a casing and reflects, at a variable angle, the beam that has passed through the optical system; and a transmissive window that is disposed in the casing at a position through which the beam passes, is made of a uniaxial crystal, and obtains a phase difference with respect to the beam passing therethrough having a wavelength λ. The phase difference is λ/4 times a positive odd integer and results from a setting of an orientation of a crystal axis and a thickness of the transmissive window.