A sub-mount includes a recess for holding a polymer optical waveguide film and recesses for embedding and holding a light emitting element and a light detecting element. The sub-mount further has an adhesive filling groove formed adjacent to respective recesses. When mounting the polymer optical waveguide film, the light emitting element, and the light detecting element onto the sub-mount, an adhesive is accurately applied and is prevented from flowing out as the adhesive filling grooves are provided.