An optical wavelength converting device is provided with a LiTaO3 substrate, a plurality of inverted-polarization layers periodically arranged in an upper surface of the LiTaO3 substrate, and an optical waveguide crossing the inverted-polarization layers. The upper surface of the LiTaO3 substrate is directed toward a -X-crystal axis direction. The inverted-polarization layers are formed by exchanging Ta ions of the LiTaO3 substrate for H ions, and an extending direction of each inverted-polarization layer is inclined at an angle of theta degrees (6 ions of the LiTaO3 substrate and the inverted-polarization layers for H ions to set a refractive index of the optical waveguide higher than that of the LiTaO3 substrate. The optical waveguide extends in a +Y-crystal axis direction. Fundamental waves polarized in a transverse electric mode induce electric field directed in +/-Y-crystal axis directions and are converted into second harmonic waves in the optical waveguide.