PURPOSE: To improve safety and reliability under a state of energy saving and instrumental saving, by a method wherein a temperature of clean air blowing off into an environmental chamber is detected and temperatures of circulating air, and taking-in outdoor air are cooled up to a necessary temperature based on a detected values of the temperature and blown off into the envioronmental chamber without reheating them.

CONSTITUTION: Air in an environmental chamber 1 is taken in the inside of an air treatment device through a blowing opening 4, and charged again into the environmental chamber 1 through a high-performance filter 6 after it has cooled by passing through a cooling coil 5. This cooling treatment is done by controlling capacity of a cooling device into a state necessary and sufficient for cooling up to a required temperature based on a detected signal of a temperature detector 8. At the time of the treatment of air, it is so constituted that outdoor air is taken in the title device through the outdoor air taking-in opening 12 and cooled up to a necessary temperature by mixing the same with circulating air. The inside of a system is kept at positive pressure always and the outdoor air is prevented from entering into the environmental chamber 1 by introducing the outdoor air in the system. Surplus air flows out of the system through a gap of a partition wall 2 and others.