Substantially S-shaped optical waveguides are embedded in a semiconductor substrate, and at least two optical waveguide returning parts are interposed between the input and output ends of the waveguides. Each of the optical waveguide returning parts includes a multiplexing portion. A reflecting part is located on a rear end side of the multiplexing portion of each optical waveguide returning part. Thus, the length between input and output ends of the waveguides can be reduced, suppressing bending loss, and achieving a high speed and small size integrated optical device.