Transmission signal light emitted from a light emitting element is coupled to an optical waveguide and guided into a transmission line. A light monitor a portion of leakage light, which is used for control of optical output of the light emitting element. The optical semiconductor module is covered with transparent resin that is closely matched to the refractive index of a clad layer covering the optical waveguide. The light leaked to the inside of the clad layer is not reflected at a top surface of the clad layer and is received at a light receiving surface of the light monitor.