To increase transmission distance with low consumption power, in a bidirectional optical transmission system, where a down CW light of constant intensity transmitted from station side equipment is modulated and returned as an up signal transmitted from user side equipment to the station side equipment.

An outgoing CW light of constant intensity is transmitted from first transmission equipment (office equipment) to second transmission equipment (user side ONU) via an optical fiber transmission line, and the second transmission equipment produces the incoming optical signal by phase-modulating the outgoing CW light and returns the signal to the first transmission equipment. It combines the outgoing CW light and the incoming optical signal received and generates interferences, thereby detecting and demodulating phase change of the phase-modulated incoming optical signal as intensity change, and conducting transmission of the incoming signal from the second transmission equipment.