PURPOSE: To enhance the heat exchanging efficiency and the reduce the number of heat exchangers by a method wherein the present cooling device is constituted so that heat exchange between compressed air after dehumidifying, which necessitates heating, and a cooling water, necessitating cooling, is effected in a heat exchanger such as a cascade condenser and the like.

CONSTITUTION: The cooling medium of a predetermined pressure or water is supplied into a water sending pipe 12, connected to a reserving tank 10, by a pump 14. The water sending pipe 12 is connected to a cascade condenser 20 whereby heat exchange is effected between the water and cooling air, supplied through a ventilating pipe 22 sending dried air discharged out of a compressed air dehumidifying device 8, and the water is cooled but the air is heated contrarily. Water, discharged out of the cascade condenser 20, is supplied to the mirror circuit of a laser processing machine through the water sending pipe 12 to cool the mirror circuit and heated water is returned to the reserving tank 10. The air, discharged out of the cascade condenser 20, is changed to air, whose relative humidity is low, and is ventilated to a desired place.