To stabilize the output of second higher harmonic of a second higher harmonic generator which is provided with a wavelength converting optical waveguide that converts fundamental waves into second higher harmonic and a reflection grating section that fixes the wavelength of the fundamental waves made incident on the optical waveguide.

A conversion substrate 2 is provided with an optical waveguide 3 which converts fundamental waves 6 into second higher harmonic 8 and a reflection grating section 5 which fixes the wavelength of the waves 6 that made incident on the waveguide 3. Moreover, this device is provided with measuring means 18A and 18B which measure the outputs of fundamental waves A1, A2 and 7 emitted from the substrate 2 and the outputs of second higher harmonics B1, B2, B3 and 8 and an output control means 20 which controls the output of the harmonic 8 emitted from the end face of the waveguide 3.