Indoor air even in negative pressure room leaked out promptly, when the door was opened. We discussed the conditions of push-pull airflow system to prevent air leakage even if the door has opened. The experiments were conducted in a full-scale test room. A push-pull device was set between an anteroom and a corridor. Baby powder was used for tracer particle. It was produced in the corridor. The concentration ratio of the anteroom to the corridor was used for effect measurement. The target value was set to less than 5 % which is equal to the exposure quantity of a person in the corridor who wears a N95 mask. The target value was achieved when the push device was set to 1000 m3/h and 0.4 or 0.8 m wide. To double the width of the push device was more effective than to double the velocity of push air when it was compared by the same air volume of the push device.