To output a laser beam having a wide stable wavelength band width.

An external resonance type wavelength variable light source apparatus comprises a semiconductor laser 21 associated with an active layer 2 having a quantum well structure, and a diffraction grating 5 for diffracting the beam irradiated from the laser and feeding back the beam 3 having a selected wavelength to the laser. In this apparatus, the laser 21 has an antireflection film 23 formed on the emitting end face 1b of the beam of at least the grating side thereof. Further, the quantum well layer 17 of the quantum well structure of the layer 2 of the laser is set to three or less layers.