A method and apparatus for measuring the optical power of very weak light arriving at a receiver, by using a photon detector, are provided. A photon detector detects the presence or absence of the arrival of a photon in accordance with bias application timing. For a train of optical pulses coming in at an arbitrary timing in respective time slots, the bias application timing is sequentially shifted within the range of a time slot. Each time a shift is made, the number of photons detected is counted by a photon counter. Based on this number of photons, the optical power of the train of the optical pulses is measured.