To provide bidirectional light transmission in a single wavelength in a light transmission system which transmits an optical signal with a passive optical fiber transmission line through the use of light transmitter and a plurality of light receivers.

The light transmitter 1 inputs an outgoing signal such as a multi-channel video signal, etc., multiplexed on a frequency axis to a semi-conductor laser 3 and, outputs the optical signal with the light frequency modulated therein (outgoing FM modulation light). The light receivers 2 multiplex the outgoing FM modulation light outputted from the light transmitter with the output of the semi-conductor laser 15 which modulates the light frequency by an incoming signal input. The receivers obtain an FM signal with a frequency being a difference between the two light frequencies after heterodyne-detection. When the FM signal is FM-demodulated by an FM demodulator 18, a signal is obtained, which is provided by frequency-multiplexing the outgoing signal such as the multi-channel video signal that is inputted to the optical transmitter with an incoming signal that is inputted to the light receivers.