An optical waveguide device includes a waveguide layer that converts a
wavelength of incident light and emits converted light. In the waveguide
layer, a ridge waveguide and slab waveguides are provided, the slab
waveguides being formed on both sides of the ridge waveguide with recess
portions intervening therebetween. The waveguide layer satisfies a
multi-mode condition for the incident light, and light propagating
through the ridge waveguide is in a single mode.