A three-dimensional optical waveguide includes plural cores, a cladding, and an optical path converting device disposed at least at a part of the cores. The cores are provided in a specified direction at least in two rows and in two layers. The cores are buried in the cladding and the cladding has a refractive index different from that of the cores. The optical path converting device converts the direction of the optical path formed by the cores to a direction different from the specified direction.