PURPOSE: To provide stable laser outputs while keeping the lighting property of an excitation lamp good by securing sufficient cooling power.

CONSTITUTION: Part of cooling water retained in a tank 1 is forced to circulate through ion exchange resin 8. The remaining amount of flow circulated by a pump 2 is fed to the excitation lamp 21 of a laser generator 4 and then the cooling water heated is cooled via a heat exchanger 5. When the temperature detected by a temperature sensor 10 provided inside the tank 1 is at or greater than a predetermined value a control unit 9 closes a solenoid valve 7 thereby reducing the amount of cooling water flowing through the ion exchange resin 8; i.e., the amount of flow of cooling water fed to the excitation lamp 21 of the laser generator 4 is increased to enhance cooling efficiency so as to restrain the temperature rise of the lamp.