PURPOSE: To achieve a light source where light intensity is stable over a short period of time and long period of time and frequency is stable by controlling frequency with current and light intensity with temperature simultaneously.

CONSTITUTION: Laser beam from a semiconductor laser 1 is divided into two directions by a beam splitter 2, one laser beam of the divided beam is allowed to inside a light-transmission element 4, and transmission light intensity and the other laser beam are converted into voltage by a second photoelectric converter 5 and a first photoelectric converter 3, respectively. Then, an output voltage V111 from the first photoelectric converter 3 and an output voltage V222 from the second photoelectric converter 5 are fed to a first comparator 6. Then, by detecting the voltage difference V222-V111 is detected, negative feedback control is made to a drive current source 7 so that output signal from the first comparator 6 is equal to 0. Also, by detecting the voltage difference Vs2s-V111 between a reference voltage Vs and an output voltage V111 with a second comparator 9, a negative feedback is made to a temperature controller 10. Thus, a light source can be obtained, where both frequency and light intensity are simultaneously stabilized.