A small size and low cost optical receiving apparatus, which can stably demodulate the signal, lights in accordance with the differential M-phase shift keying (DMPSK) system. An optical receiving apparatus comprises a light branching unit for branching the differential M-phase shift keying signal beam into four signal light beams, a delay adjusting unit giving a delay time difference of one symbol between a first signal light beam and a second signal light beam and between a third signal light beam and a fourth signal light beam, a demodulating unit for outputting a least two demodulated light signals through respective interferences between the first signal light beam and the second signal light beam, and between the third signal light beam and the fourth signal light beam on one plane which is not parallel to any signal light beam, and at least two optical detectors for converting at least two light signals into electrical signals.