A light amplifier includes first and second multi-pass amplifiers, an excitation light source, and a beam splitter. The second multi-pass amplifier includes a light attenuation portion provided in an optical path for a light pulse to travel to pass through a light amplification medium a plurality of times, for attenuating energy of the input light pulse. In addition, an excitation light pulse from the excitation light source is split by the beam splitter into two light pulses. These two pulses are input to the first and second multi-pass amplifiers, respectively. Thus, fluctuation in energy of the light pulse output from the light amplifier can be less than fluctuation in energy of the excitation light pulse.