The present invention relates to a laser light source having a structure for narrowing a wavelength bandwidth of output light. The laser light source comprises, at least, a laser resonator, a pumping light source, an optical path switch device, a diffraction grating, and a total reflection mirror. The laser resonator has a light amplifying fiber for output of emission light arranged on a resonance optical path thereof. The optical path switch device has a first port and a second port on the resonance optical path of the laser resonator. The diffraction grating spectrally separates the emission light outputted from the second port. The total reflection mirror reflects a specific wavelength component out of wavelength components spectrally separated by the diffraction grating, so as to feed the specific wavelength component back to the second port. Particularly, the optical path switch device controls a light transmittable state of the emission light outputted from the light amplifying fiber, at the second port.