A light source apparatus includes an optical resonator formed by an optical amplification medium and an optical switch. The optical switch includes a saturable absorber and changes its transmittance or reflectance when receiving an optical pulse emitted from a light irradiation source which includes a wavelength-tunable light source. The light source apparatus emits amplified light from the optical resonator in correspondence with the center wavelength of the optical pulse from the wavelength-tunable light source. The relationship between a length L and an effective refractive index n of the optical resonator and a repetition frequency f of the optical pulse satisfies a condition L (nL)/c.