PURPOSE: To reduce time for restoring a gain in an optical communication system of a specific optical data bit speed by making a carrier storing area adjacent to a gain area, so as to rapidly increase carrier density within the gain area. CONSTITUTION: Time for restoring a gain in an optical communication system of more than 1GHz bit rate of an optical data bit stream is reduced by making the carrier storing area 12 located adjacent to the gain area 10. Then the time in which the carrier moves from an area 12 to the area 10 is set to be shorter than the true life time of the area 10. This is attained by rapidly increasing the carrier density within the area 10, by generating the diffusion of the carriers from the area 12 to the area 10, by forming carrier density gradient between the areas 12 and 10 just after an optical signal has passed through the area 10. Thereby time of recovery of a gain is reduced and enables rapid recovery of the gain of an amplifier.