When one tries to obtain high-output wavelength conversion light of class W, the internal temperature of a wavelength conversion element rises significantly to cause an undue variation in phase matching wavelength and thereby it is difficult to match the wavelength of a fundamental wave to the phase matching wavelength. A part of the output of the fundamental wave or pumping light propagating on a fiber is thereby absorbed by doping the fiber with a rare earth element at a set concentration, and the temperature of a fiber grating is raised by heating the fiber grating with heat produced through the absorption. Consequently, the wavelength of the fundamental wave obtained through the fiber grating by widening the grating interval is substantially matched to the phase matching wavelength which can be converted by the wavelength conversion element, and a wavelength converter ensuring a stable high output up to class W is provided.