A light amplifying device includes, a light amplifying fiber to amplify a seed beam from a seed light source with an excitation beam from an excitation light source, and a controller. The controller causes the seed light source to emit the seed beam in an emission period in which the light amplifying fiber outputs the amplified light beam, causes the excitation light source to emit the excitation beam having first-level power in a non-emission period immediately before the emission period, and changes the excitation beam power to a second level higher than the first level at a beginning of the emission period. The controller increases the excitation beam power to a third level higher than the second level after starting of the emission period, and decreases gradually the excitation beam power from the third level to the second level.