PURPOSE: To attain accurate two-way communication without causing reduction in a transmission capacity or reduction in the degree of freedom of usage.

CONSTITUTION: A transmission section 1 emits an optical signal with a predetermined polarized angle into space. A 1st optical system 21 and a 2nd optical system 24 transnrit an incoming light with an almost equal transmittivity as to an optical component of a polarized angle of an optical signal emitted by its own transmission section 1 into space and a different transmittivity as to an optical component of a polarized angle of an optical signal emitted by a transmission section 1 of an opposite communication party (the latter polarized angle differs from the polarized angle of an optical signal emitted by its own transmission section 1). A 1st light-current conversion circuit 22 generates an electric signal ES3 in response to the luminous quantity of the transmitted light from the lst optical system 21 and a 2nd light-current conversion circuit 25 generates an electric signal ES4 in response to the luminous quantity of the transmitted light from the 2nd optical system 24 respectively and a differential amplifier circuit 23 calculates a level difference between the electric signals ES3 and ES4.