PURPOSE: To contrive to stabilize laser output over a wide output range by controlling an exciting unit and a Q-switching driver so as to make constant the ratio of excited energy to an oscillation threshold value.

CONSTITUTION: A laser medium 3, a polarizer 5, and an electro-optical element 6 are placed in a photo resonator consisting of reflections mirrors 1 by fitting optical axes of laser lights. The exciting unit 4 which excites the laser medium 3 and the Q switching driver 7 which drives the electro-optical element 6 start the operation by a signal from a pulse generator 9. A λ/4 plate 11 is provided between the polarizer 5 and the electro-optical element 6, and the laser output is taken out as the reflected light of the polarizer 5. A control unit 10 monitors the laser output energy and then controls the exciting unit 4 and the Q-switching driver 7 so that the ratio of laser output energy/laser oscillation threshold value becomes constant.