An optical wavelength converting apparatus comprises a fundamental wave source, which produces a laser beam serving as a fundamental wave, and an optical wavelength converting device, which is provided with an optical waveguide constituted of a nonlinear optical material. The optical wavelength converting device converts a wavelength of the laser beam impinging upon and guided through the optical waveguide. The optical waveguide is provided with a plurality of periodic domain-inverted structures, which effect phase matching between the laser beam and wavelength-converted waves. Reflection of the fundamental wave or wavelength-converted waves from end faces of the optical wavelength converting device is thereby minimized, and a plurality of wavelength-converted waves having high intensities are obtained in a state combined with one another.