A semiconductor laser device according to the present invention outputs light while periodically varying the temperature of a semiconductor laser, an optical waveguide, or a diffraction grating, outputs light while causing the optical waveguide or the diffraction grating to mechanically and periodically vary, or causes return light, which varies periodically or non-periodically, to be incident on the semiconductor laser. Since the periodical fluctuation is applied to the temperature of the semiconductor laser, the optical waveguide, or diffraction grating, the periodical mechanical variation is applied to the optical waveguide or the diffraction grating, or the return light is caused to be incident on the semiconductor laser, the semiconductor laser carries out a multimode oscillation from a low output to a high output without shifting between a single mode oscillation and a multimode oscillation. Accordingly, the present invention can provide a semiconductor laser device having IL characteristics in which a current is proportional to a light output and a driving method thereof.