The optical switch comprises a loop-shaped light guide loop circuit 101 made of an optical nonlinear medium, a control light input means 20 of a phase control means 110 inputting control light in thelight guide loop circuit, a wavelength separating/multiplexing circuit 121 and a phase bias circuit 500. The light guide loop circuit is constituted of an optical path made of a first polarization surface preserving optical fiber 12 from an optical demulti/multiplexing device 10 to the control light input means 20, an optical path made of a second polarization surface preserving optical fiber 14 from the control light input means to the wavelength separating and coupling circuit and an optical path made of a fourth polarization surface preserving optical fiber 18 from the wavelength separatingand circuit to the optical demulti/multiplexing device. The phase bias circuit is inserted in the middle of any one of the first to the fourth polarization surface preserving fibers.