There is provided a multimode optical amplifier that provides gain to a plurality of propagation modes of signal light. The multimode optical amplifier includes a multiplexer that multiplexes the signal light and excitation light; an amplifying fiber which has a core that propagates a predetermined plurality of propagation modes of the signal light and a predetermined plurality of propagation modes of the excitation light, and which provides an individual gain value for each of the predetermined propagation modes of the signal light; a wavelength-flattening filter that maintains a constant gain value for a frequency for all predetermined propagation modes of the signal light; and a mode-flattening filter that maintains the gain value at a constant value for a predetermined propagation mode of the signal light for all predetermined propagation modes of the signal light.