The present invention provides an optical switch whose operational characteristics do not change, and which is not affected by polarization cross talk, even when the wavelength of a signal light or the ambient temperature of the optical switch varies. The optical switch comprises first and second polarization splitting/combining modules, (10,18), first and second polarization-maintaining single-mode fibers (12, 16), a first polarization plane conversion portion (14), third and fourth polarization-maintaining single-mode fibers (22, 26), and a second polarization plane conversion portion (24).