PURPOSE:To quickly remove the dust that fills a compartment so that the duty may not enter a clean room by discharging a strong jet of clean air when entering the clean room, which is continued to be supplied at a suitable airflow velocity for several seconds immediately after terminating the jet. CONSTITUTION:The light wind air cycle (I) draws the air in a compartment 11 by a light wind fan 12, and blows same into the compartment 11 at a rate of 0.05-0.1m/s through a prefilter 13 and a ultra high-performance filter 14. Such a circulating operation is normally continued. The air jet (II) discharges the air in the compartment 11 into the compartment from air jet nozzles in a ventilating side wall 115 at a rate of 25m/s by the main fan 16. Such a circulating operation is conducted for 10-20sec only when a person entered into the compartment 11. The laminar airflow (III) supplies the air in the compartment 11 into the compartment at a rate of 0.5-1m/s by the main fan 16 as a vertical laminar flow from the ventilating ceiling 113. Such a circulating operation is conducted for 5-10sec from immediately after stopping the air jet to the door opening.