A planar light wave circuit is disclosed, which comprises: a first waveguide for receiving optical signals from an outside; a second waveguide having a first end surface and a second end surface, so that lights outputted from the first waveguide are partially reflected by the first end surface and are partially incidented into the first end surface and then outputted through the second end surface; a third waveguide for receiving the light reflected by the first end surface; and a common region bordering each end surface of the first to the third waveguides and having an index of refraction different from an index of refraction of the second waveguide.