PROBLEM TO BE SOLVED: To provide an optical waveguide which is downsized and in which the diameter of an optical fiber on a light emission side can be reduced by converting the core diameter of the light emission side.SOLUTION: An inventive optical waveguide is provided with a multiplexing core which multiplexes three or more beams made incident from light incident part sides and propagates them to a light emission side. Among the three or more light incident parts, at least one light incident part is at a height different from that of the other two or more light incident parts which are at the same height, and the multiplexing core has a taper shape part of which breadth and thickness decrease from the light incident part toward the light emission part.