PURPOSE: To accurately conduct the AGC of an optical amplifier by correctly detecting the gain of the optical amplifier.

CONSTITUTION: A wavelength branched optical filter 12 is provided on the latter stage of a optical-amplification body 11, only the natural emission light component PSP', which is discharged by the optical amplification medium 11 itself from the output light of the optical amplification medium 11, is branched and it is inputted to an O/E light detector 13. The natural emission light component PSP', which is discharged by the above-mentioned optical amplification medium 11 itself, is in proportion to the gain G of the optical amplification medium 11. A return circuit 15 returns to the optical amplification medium 11 in such a manner that the detection signal corresponding to the above- mentioned natural emission light component PSP' and a reference signal Ref, l will be made equal. Accordingly, as the gain G is accurately detected and it is controlled properly, a correct AGC can be accomplished.