An optical amplifier of the present disclosure comprises an optical resonator which includes an amplifying fiber capable of amplifying signal light including one or more propagation modes, and which causes at least one propagation mode included in the signal light amplified by the amplifying fiber to resonate; an excitation light source which outputs excitation light for exciting the amplifying fiber; and a multiplexer which multiplexes the signal light and the excitation light. The optical resonator includes a gain clamp setting unit which sets a gain clamp with respect to at least one propagation mode among a plurality of propagation modes that resonate in the optical resonator.