An optical waveguide device includes: a waveguide core that guides light; a mirror surface that deflects light coming from the waveguide core by 90°; a main waveguide core that guides light deflected at the mirror surface; a waveguide core for monitoring that branches the light deflected at the mirror surface off from the main waveguide core, and guides the light in a different direction, the mirror surface being disposed at a branching portion of the waveguide core for monitoring; and a clad portion that surrounds the waveguide core, the main waveguide core and the waveguide core for monitoring.