PURPOSE: To prevent the positional slippage of an external mirror due to the non-uniform increase in temperature of a substrate by a method wherein a temperature sensor is installed in the neighborhood of a mount holding the laser tube of an external mirror type laser device, and the temperature of each section is maintained at a constant level by controlling a ventilating means.

CONSTITUTION: The laser tube 1 is held by mounts 2a, 2b, and 2c, being placed on the substrate 3 along with external mirrors 4a and 4b and contained in a case 5. The side wall of the case is provided with an air inlet 6 and air outlets 7a, 7b, and 7c having fans 8a, 8b, and 8c. When a laser beam 9 is generated by the application of voltage on the laser tube 1, the center of the tube 1 comes to the state of the highest temperature, and the substrate 3 the highest temperature in the neighborhood of the central mount 2b. The temperature of each mount is detected by thermistors 10a, 10b, and 10c, and a mount with increased temperature is cooled by the rotation of a fan corresponding via a control circuit 3, resulting in the uniformity of the temperature distributions of the substrate 3 and each mount.