The invention is directed to an optical waveguide that reduces an excess loss caused in a curved waveguide region by a deviation of the center axis of a beam propagating mode from the center axis of the optical waveguide. The optical waveguide has its part curved, and assuming that the shortest distance from a certain point of a convex edge of the curved portion to a concave edge is a waveguide width at that point, the optical waveguide has its width progressively reduced from the maximum waveguide width in the midst of the curved portion toward the opposite ends of the curved portion.