The present invention relates to a light source apparatus having a structure to enable a short pulse optical output with high pulse energy or a high pulse peak equivalent to that of a Q switch laser light source. The light source apparatus comprises a seed light source, a pre-stage optical amplifier, a subsequent-stage optical amplifier, and a control section. After the pulse light outputted from the seed light source is amplified by the pre-stage optical amplifier, the pulse light is amplified further by the subsequent-stage optical amplifier. In the subsequent-stage optical amplifier, a ring-type resonator is constituted by an optical coupler, an optical coupler, an optical amplification waveguide, an optical isolator, an optical coupler, an optical waveguide, a lens, a Q switch, a lens, and an optical waveguide that are disposed sequentially on the propagation path of the pulse light. The control section controls a Q switch such that the resonator loss of the resonator in the subsequent-stage optical amplifier is minimal with the timing at which the pulse light is inputted to the subsequent-stage optical amplifier.