To eliminate the need for an optical filter, to simplify constitution and to obtain large conversion efficiency by short length in a wavelength con verter for converting the wavelength of signal light by non-degenerated four- optical wave mixing.

The wavelength converter is provided with a resonator constituted of opposing a reflector 21 for reflecting only light included in a 1st wavelength band and transmitting light other than the 1st wavelength band to a reflector 22 for reflecting only light included in a 2nd wavelength band partially superposed to the 1st wavelength band and transmitting light other than the 2nd wavelength band and a gain medium 23 arranged between both the reflectors 21, 22 and constituted so that the center wavelength of gain is included in a wavelength band in which the 1st and 2nd wavelength bands are mutually superposed and the size of the gain exceeds an internal loss and signal light 29A of a specified wavelength transmitted through the reflector 21 and reflected by the reflector 22 is made incident from the reflector 21 side.