In a planar
waveguide optical circuit 1, a inclined groove 3 is formed at an inclination angle .theta. with respect to the
vertical axis so as to cross optical waveguides 2.sub.n. A reflection filter 4 structured such that the difference in
reflectivity between orthogonal polarization is compensated for with respect to
signal light is installed inside the groove 3, the reflected light from the reflection filter 4 is detected by the photodetectors 6.sub.n, and the
optical intensity of the
signal light is monitored. This makes it possible to accurately monitor the
optical intensity regardless of the polarization state of the
signal light. Also, since the inside of the groove 3 including the reflection filter 4 is sealed with a filler resin 5, any deterioration in long-term stability that would otherwise be caused by
contamination of these components is prevented. Thus, the optical
waveguide module with which the structure of the optical circuit is simpler, and the
optical intensity can be correctly monitored regardless of the state of polarization of the
signal light is realized.