PURPOSE: To provide an optical frequency conversion method by which a conversion operation having no dependency on polarized waves is made possible in frequency conversion using four light waves mixing of a semiconductor optical amplifier and to provide a device therefor.

CONSTITUTION: Signal light is multiplexed with the pumping light of two orthogonal polarized waves having equal intensity from a pumping light source 10 and the multiplexed light is inputted to a first terminal of a polarization beam splitter 12. The output light from a second terminal of this polarization beam splitter 12 is inputted to a first terminal of the semiconductor optical amplifier 13 having a gain characteristic without having the dependency on the polarized waves in the state of maintaining the polarized waves. The polarized waves are maintained in the output light from a third terminal of the polarization beam splitter 12 and the polarized waves orthogonal with the polarized waves inputted to the first terminal are inputted to the second terminal facing the first terminal of the semiconductor optical amplifier 13.