PURPOSE: To make the amount of light emission constant for a light emitting element and to improve the ability of optical communication through a detection circuit for light amount provided at one end of an optical fiber directly connected from a photodetection circuit at the remotest location to a light transmission circuit.

CONSTITUTION: Photodetection circuits LRX1∼LRXn are connected to a light emission element LEM connected to a current source LC via the 1st optical fiber OF1 and a branch coupler DVC. The 2nd optical fiber OF2 is connected to the coupler DVC of the photodetection circuit LRXn at the remotest end in the optical communication system with this constitution, and a light amount detection circuit LDT is connected to another end of the optical fiber OF2. The current source LC is controlled with the detected output of the detection circuit LDT to keep the light emission amount of tge LEM constant, a stable light transfer can be made, the extension in the photodetection circuit can be made easy to increase the ability of light communication.