PURPOSE: To prevent the infiltration of polluted outside air and pollutant attached to a car by combination of jet current blow off units for delivering air in a parking lot toward the entrance and an air blower for blowing it inclined outside at the entrance thereof.

CONSTITUTION: Jet current blow-off units 5aW5e for delivering air are mounted close to the ceiling of a parking lot to deliver ambient air of a jet current in that order toward the entrance 6 thereof in one direction. An air blower, set up at the entrance thereof, receives air being delivered at a receiving port 7 and blows off a fast air inclined outside from slits 9a, 9b, 10 and 11 on two side walls, on the ceiling surface and on the floor surface after boosted to a high pressure with fans 8 and 8'. A differential pressure happens to be generated between inside and outside the parking lot with the operation of the blower. Differential pressure detectors 12 are mounted inside and outside the blower to perform a gradual control of a fan 8 according to a detected value.