PURPOSE: To generate extremely short pulse beams in narrow frequency range to be actuated in extremely high stability.

CONSTITUTION: An accessory outer resonator 110 comprising a semiconductor phase fluctuating part 111 with one surface thereof coated with a non-reflecting film 112 is annexed to a main resonator 100 while output short pulse beams are branched to be photoelectrically converted. Next, the position of an outer translucent mirror 121 is fluctuated by the detection signals of the photoelectric conversion so that the intensity of the output short pulse beams may be miximized as well as the length of the main resonator 100 may be made an integer fraction of the length of the accessory resonator 110 in the space range precision of the output short pulse beams for adjusting the length of the main resonator 100. Through these procedures, the title short pulse semiconductor light source generating conventionally unattainable extremely short pulse beams in narrow frequency range actuating the beams in extremely high stability can be realized.