PURPOSE: To automatically keep the pressure in a flow-rate metering passage at a prescribed pressure by means of a simple constitution by controlling the normal- reverse revolution of an exhaust blower, according to the differencial signals between detection signals and prescried signals.

CONSTITUTION: A pressure sensor 3 equipped with a pressure sensing port 2 is formed on the side wall of a flow-rate metering passage 1 and connected with a comparator 5, with a prescribed-signal generator 4 arranged separately, and the comparator 5 is connected with the reversible motor 7 of an exhaust blower 6. Thus, the pressure in the passage 1 is converted to DC voltage or current by the pressure sensor 3, and in comparator 5, compared with the standard voltage or current from the prescribed-signal generator 4 constituted of a constant voltage generator and a potential divider, and according to the deviation signals, the reversible motor 7 revolved normally or reversely.