A light source device can attain a stable output of a harmonic even when there occurs a change in the ambient temperature or fluctuation in the output power. The light source device is provided with a semiconductor laser source (4), an optical waveguide-type QPM-SHG device (5) for generating a second harmonic from light emitted from the semiconductor laser source (4), a wavelength control means (7) for controlling a wavelength of light emitted from the semiconductor laser source (4), a means for slightly fluctuating wavelength (8) for changing a wavelength of light emitted from the semiconductor laser source (4) and a means for detecting a change in output light power of the optical waveguide-type QPM-SHG device (5) that occurs when a wavelength of light emitted from the semiconductor laser source (4) is changed. In this case, a wavelength of light emitted from the semiconductor laser source (4) is controlled to an optimum wavelength of the optical waveguide-type QPM-SHG device (5) based on a change in output light power of the optical waveguide-type QPM-SHG device (5) that occurs when a wavelength of light emitted from the semiconductor laser source (4) is changed.