An optical amplifier includes: a first amplifier amplifying a signal light by a first excitation light; a variable optical attenuator attenuating the signal light; a second amplifier amplifying the signal light by a second excitation light; a mode selector selecting one of first and second modes; a gain controller, in first mode, controlling first and second excitation lights so that a gain of power of the signal light becomes constant; a first output controller, in second mode, controlling the first excitation light; a second output controller that, in second mode, controlling the second excitation light so that a spontaneous emission light having fixed level is outputted; and an attenuation controller controlling an attenuation of the variable optical attenuator according to an input level of the signal light in first mode, and controlling the attenuation to become a given value larger than a value of first mode in second mode.