PURPOSE: To avoid operation of a blower or exhauster in the surging range so as to prevent damage to the fan, etc., in each of them by providing a bypass valve in a bypass line provided between the air-supply equipment and the exhaust equipment in a setup to have the valve opening of the bypass valve adjustable.

CONSTITUTION: A bypass line 15 is provided by connecting one end to a point on the upstream side of a plurality of separation valves 7a, 7b installed in an air-supply line to a secondary containment plant area 8 and by connecting the other end to a point on the downstream side of a plurality of separation valves 7c, 7d installed in an exhaust line from the secondary containment plant area 8. A bypass valve 16 is installed at an intermediate point in the bypass line 15. The valve opening of the bypass valve 16 is controlled in a manner of making air flow at a pressure and at a flow rate which do not involve surging caused by blowers 4a, 4b, 4c and exhausters 12a, 12b, 12c. As a result, the blowers 4a, 4b, 4c and the exhausters 12a, 12b, 12c are kept from operating in the surging range and damage to the fans can be prevented.