This invention is applied to an optical amplifier evaluation method of modulating by an optical modulator 23 light output from a light source 1 into a rectangular optical signal which is enabled/disabled in a predetermined period, then applying the optical signal to an optical fiber amplifier 2 to be measured, and obtaining the gain and noise figure of the optical fiber amplifier from the light intensities in the ON and OFF periods of an optical signal output from the optical fiber amplifier and the light intensity in the ON period of an optical signal input to the optical fiber amplifier. Output light from the optical fiber amplifier in a no-input state is passed through an optical path extending from the light source to the optical fiber amplifier and an optical path extending from the optical fiber amplifier to a light intensity measurement position, thereby obtaining optical losses on the respective optical paths. By the obtained optical losses, the light intensities are corrected. As a result, the gain G and noise figure NF of the optical fiber amplifier 2 can be attained with high precision.