A wavelength-variable light source apparatus having a wavelength calibration function of emitted light therefrom. The apparatus includes a semiconductor laser light source section, an external resonance section having a reflection surface for externally resonating emitted light from the semiconductor laser light source section at a predetermined reflection wavelength, a drive section for moving a position of the reflection surface in the external resonance section for varying an external resonance condition, a control section for controlling a variable move distance of the reflection surface in the drive section, and a wavelength detection section comprising a gas cell having a wavelength absorption characteristic absorbing light at a plurality of specific wavelengths for sweeping light emitted from the external resonance section into the gas cell and detecting an absorption wavelength of the emitted light, wherein the control section controls the move distance of the reflection surface in the drive section based on the absorption wavelength detected by the wavelength detection section.