PURPOSE: To prevent breakage of a cooling-water passageway vessel and to stabilize a laser output by maintaining the temperature of cooling-water drainage below the prescribed temperature irrespective of variations in temperature of city water and in heating power of a laser tube.

CONSTITUTION: In case the temperature of a cooling water drainage 8' is below the prescribed temperature, a cooling water temperature controlling circuit 14 is operated by a detection signal from a temperature detector 13 and electromagnetic valves 12 and 13' are closed. Therefore, the cooling water in a cooling water mixing vessel 15 becomes a pressurized cooling water 16 by the operation of a water pump 11, the generated heat at a laser fine tube 9 is absorbed in a cooling water passageway vessel 10 and the cooling water is discharged into the vessel 15 as drainage 8. When the temperature of the drainage 8 exceeds the prescribed temperature while repeating a heat absorption, a circuit 14 starts operation by receiving the detection signal from the detector 13, an "ON" position is given to the electromagnetic valves 12 and 12' and city water 7 passes through the electromagnetic valve 12 and flows in the vessel 15 as city water 7', thereby reducing the cooling water temperature in the vessel 15 and maintaining the temperature of the drainage 8', which absorbed the heat generated at the laser tube 9, below the prescribed value. Thus, the temperature of the cooling water 8 is maintained at the prescribed temperature and below by the repetition of "ON" and "OFF" of the electromagnetic valves 12 and 12'.