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.