An optical transmission system comprises master unit and remote unit, the master unit includes optical transmitter transmits the optical signal to the optical fiber at a fixed averaged output level, the remote unit includes optical receiver receives optical signal via optical fiber, and optical receiver includes light receiving element converts optical signal into electric signal, variable gain amplifier amplifies the electric signal at a gain corresponding to a value of a given adjustment voltage, gain controller supplied power for, a defined control period, controls the adjustment voltage to stabilize an averaged level of the electric signal to a defined value, and feed-forward-controls a gain of variable gain amplifier, a shut off unit shuts off supply of power to the gain control circuit when the control period ends, and a keeping unit keeps a value of an adjustment voltage applied to the variable gain amplifier.