PURPOSE: To cool a solid laser medium efficiently and firmly and to enable the laser processing with a high output for a long time by arranging through holes for cooling.

CONSTITUTION: The holes 3a, 3a for cooling are formed in a laser medium 3 with penetrating through it in an axis direction of laser oscillation. The excited light rays from an excitation lamp 4 are gathered by condensers 1 and 2 thereby increasing the energy of the light, after which the light rays are projected on the laser medium 3 to cause it to pulse-oscillate or continuously oscillate the laser. In this case, as a cooling medium such as cooling water circulates or so in the holes 3a, 3a, the increase of temperature of the laser medium 3 can be restrained to the minimum.