PURPOSE: To provide the optical transmission system capable of accepting the coherent light of optically FSK-modulated signal as a signal with a band as narrow as a direct demodulated signal.

CONSTITUTION: An optical receiving device 300-1 consists of a light detecting device 1, clock extraction circuit 2, bit discrimination circuit 3, phase error detection circuit 4, and local light emitting source 5. The clock extraction circuit 2 extracts a clock signal 303 showing the timing which discriminates the bit from a detection signal 302 and send it to the bit decision circuit 3. The circuit 3 judges whether or not the phase of the received optical FSK signal 102 is advanced or delayed by taking the phase of a local light emitting signal 301 as a reference based on the polarity of the detection signal 302 in the timing shown by the clock signal 303 and outputs the decided result as a demodulated signal 306. The optical receiving device does not require an electronic circuit equipped with the wide band exceeding the bit rate. Thus, the high speed processing can be performed and the receiving device can be made by inexpensive electronic components.