PURPOSE: To stabilize the wavelength of a narrow band laser light without necessity of a new reference light source.

CONSTITUTION: The narrow band laser comprises a laser medium 1 using fluorine, a diffraction grating 3 for band-narrowing a fluorine molecule laser light, a first resonator 2 for oscillating the laser light, a second resonator 5 for oscillating a fluorine atom laser light, a spectroscope 6 for measuring absolute wavelength of the fluorine molecule laser light by simultaneously introducing the fluorine molecule laser light and the fluorine atom laser light and detecting a deviation from a set wavelength, and a diffraction grating and peak splitter 7 for controlling the grating 3 according to the input signal from the spectroscope 6 and correcting the deviation of the wavelength. Since the fluorine atom laser light having a wavelength of a visible area can be oscillated simultaneously with the fluorine molecule laser of the narrow band laser light, the fluorine atom laser light can be used as a reference light, and a wavelength can be stabilized without necessity of a new reference light source.