PURPOSE: To allow the reproducibility of good working by allowing the gas to flow to the condenser lens, cooling, detecting the environment of this lens room with a sensor and controlling the inflow and outflow of the gas so as to make to the environment set preliminarily.

CONSTITUTION: The heat deformation and the heat degradation for the condenser lens 2 of the laser beam 1 is prevented. For this method, the double room is made with a glass parting plate 4 and the gas guiding plate 5 respectively on the upper and the lower sides of the condenser lens 2. By introducing the gas to one side room, through a small hole 8 of glass (gas guiding plate) 5 at the boundary of the room having the lens 2, flowing to the condenser lens 2, it is cooled. The environment composed of temperature, humidity, wind volume and pressure is detected with the sensor 7 set on this lens room. And the flowing in and out of the gas is controlled so as to attain the environment set preliminarily. Therefore, the heat deformation and heat degradation of the condenser lens can be prevented.