<p>There is disclosed a small-size unidirectional optical power monitor having an excellent directivity characteristic. The monitor includes: a GRIN lens having a tap film for transmitting/reflecting an optical signal from an input optical fiber and an output optical fiber with a predetermined ratio; and a photodiode having a lens for detecting/measuring a portion of the optical signal which has transmitted through the tap film. The tap film of GRIN lens is connected to the photodiode by an opaque black sleeve having continuous two circular holes serving as an internal optical path between them. The two circular holes are parallel to each other, their center axes are shifted from each other, and mutually continuous by a communicating hole at the sleeve intermediate position. The portion of the optical signal from one of the optical fibers which has transmitted through the tap film passes through the two circular holes and the communicating hole and is detected by the photodiode. One of the circular holes has a vertical wall at the sleeve intermediate position and the optical signal from the other optical fiber which has transmitted through the tap film is blocked off by the wall and not measured by the photodiode.</p>
