Abstract: A light source which is stabilized in frequencies at uniform intervals comprises a light source 5 whose light frequency is varied with a signal from a reference signal source 4; a frequency detector 6 for detecting the frequency of the output of the light source 5, the cycles of the said frequency detector being the same as the uniform intervals; a comparator 2 for comparing the output of the frequency detector 6 and the output of the reference light source 1; and a switch 3 having as its input the output of the comparator 2 for inputting the output to the light source 5, said switch being so operated that when the frequency of the light source 5 is to be changed the switch is opened and the output of the reference signal source 4 is changed to set the output frequency of the light source 5 near the desired frequency and then the switch 3 is closed.