To enable wavelength conversion near to signal light by operating even to the high speed signal with a wavelength converter utilizing the gain suppression phenomenon of a semiconductor optical amplifier and outputting only the wavelength conversion light without using an optical filter.

A semiconductor optical amplifier region is inserted into two arm waveguides 13a, 13b of a symmetrical Mach-Zehnder interferometer. Intensity modulation signal light of a wavelength λs and stationary light of a wavelength λc are inputted from different ports. The stationary light of the wavelength λc is complementarily modulated by the intensity modulation signal light of the wavelength λs in the semiconductor optical amplifier region and is outputted as the wavelength conversion light to cross ports countering the input ports.