A wavelength stabilized light source includes a laser diode which emits a laser beam of a wavelength of 1.56 .mu.m. A second-harmonic generator generates a second-harmonic light of the laser beam. The second-harmonic light enters a resonator which includes a resonance cell in which .sup.87 Rb atoms are filled. A phase-modulated microwave is irradiated on said resonator. The reasonator outputs a double resonance signal when the wavelength of the second-harmonic light is equal to a resonance wavelength of .sup.87 Rb and the frequency of the microwave is equal to a transition frequency of .sup.87 Rb. A first control circuit controls the microwave so that the frequency of the microwave is equal to the transition frequency. A second control circuit controls the laser diode so that the wavelength of the second-harmonic light is equal to the resonance wavelength of .sup.87 Rb. Thereby, the wavelength of the emitted laser beam is stabilized at 1.56 .mu.m.