Two optical waveguides and an insulating film provided to cover the optical waveguides are formed over an insulating layer. Two wirings and a heater metal wire are formed over the insulating film via an insulating film different from the above insulating film. The latter insulating film is thinner than the former insulating film, and has a higher refractive index than the former insulating film. The leaked light from either of the two optical waveguides can be suppressed or prevented from being reflected by any one of the two wirings, the heater metal wire, and the like to travel again toward the two optical waveguides by utilizing the difference between the refractive indices of the two insulating films.