<p>A light source device equipped with an oscillator (wavelength converting means) (2) which receives a pump light (L1) from a pump light source unit (1) and generates a signal light and an idler light by the optical parametric oscillation (oscillation according to pseudo phase matching method), characterized in that the oscillator has a LiNbO3 crystal doped with MgO (3) as the wavelength converting crystal thereof. The LiNbO3 crystal doped with MgO (3) has a periodic polarization reversion structure for pseudo phase matching whose polarization reversion period is such that renders possible to generate a light having an absorption wavelength of the gas to be detected at a crystal temperature lower than 100 DEG C. The above polarization reversion period is significantly different from conventional polarization reversion periods of a LiNbO3 crystal known for various gases. The light source device for use in detecting a gas requires no large sized heating apparatus for preventing the optical damage of a wavelenght conversion crystal and thus is compact, and also is excellent in explosion protection.</p>
