Abstract: Light relative to non-modulated output light emitted from a semiconductor laser is applied to an absorption cell. Frequency modulating signals are converted into amplitude modulation signals of the output light through a modulating means by utilizing absorptive properties of the absorption cell. Transmissive light outputs of the absorption cell are synchronously rectified by a lock in amplifier, and an electric current supplied to the semiconductor laser is controlled so that the output signals of the lock in amplifier are arranged to be a predetermined value. The wavelength of the output light is locked to the absorption line of the standard substance in the absorption cell. Advantageously, the invention stabilizes the instantaneous value of the oscillation frequency.