PURPOSE: To attain an inexpensive bidirectional light transmission system small in the number of component parts and occupying area by using a light emitting element and a photodetecting element to have direct connections among a light transmitter, a light receiver and a light multiplexing/demultiplexing unit.

CONSTITUTION: The electric signal supplied through an electrical input terminal 32 is converted into an optical signal of wavelength λ1 by a light emitting element 33 of a light transmitter 31. This optical signal is supplied from a light transmission/reception part 38 and outputted through a light transmission/reception part 40 by the function of a light multiplexing/ demultiplexing unit 37. While the optical signal of wavelength λ1 outputted from the part 40 is made incident on a photodetecting element 36 via a light transmission/reception part 39 by the function of the unit 37. This optical signal is converted into an electric signal by a light receiver 34 and outputted through a terminal 35. Thus bidirectional light transmitters 50 and 51 are used with the wavelengths of these transmitters set at λ1 and λ2 respectively. Then the electric signal supplied through an input terminal 52 of the transmitter 50 is supplied as an optical signal to the transmitter 51 from a light transmission/reception part 56 through a fiber 58 and a light transmission/reception part 57 then outputted through a terminal 55 as an electric signal. While the electric signal supplied from a terminal 53 is turned into an optical signal by the transmitter 51 and then converted again into an electric signal by the transmitter 50 to be delivered through a terminal 54. Then optical signals of wavelengths λ1 and λ2 are multiplexed.