PURPOSE: To send/receive a 2-way optical signal with less loss and to economize the transmitter with only one optical fiber amplifier.

CONSTITUTION: One optical fiber amplifier is provided to the transmitter. Optical isolators 131, 182 not depending on polarized light connected to an input of a forward path optical signal 121 and the input of an optical filter 192 along a forward direction of each optical signal, and optical isolators 181, 132 not depending on polarized light connected to the input of an optical filter 191 and an input of a return path optical signal 12. along a forward direction of each optical signal are used to avoid effect of a reflected return light onto the optical signal. An optical multiplexer/demultiplexer 141 inserted between one terminal of the optical fiber amplifier and the optical isolators 131, 182 and an optical multiplexer/demultiplexer 142 inserted between other terminal of the optical fiber amplifier and the optical isolators 181, 132 are used to multiplex a transmission optical signal and to demultiplex the received optical signal.