PURPOSE: To vary suction air windages between rooms in response to using state and to suitably ventilate them by sending a signal to control means by detecting variation in the using state of the room by a sensor, and moving a damper by the control means to regulate the opening area of a suction port.

CONSTITUTION: A variable resistor 10 is regulated by considering the sizes of a bathroom A and a lavatory B in which a main suction port 1A communicates with a sub suction port 1B at the time of installation and building state of a suction duct 3. Either light emitting side 7g or 7k of first and second photocouplers is radiated according to amplitude relationship between a signal E of a movable piece 10a of the resistor 10 and a signal F of a position detection piece 11 in a damper unit 7 for regulating windage at the side 7g or 7k of first or second photocoupler, a motor 7a is so rotated that the signal F becomes equal to the signal E. The opening area of the port 1A is regulated to regulate the ratio of suction air windages of the port 1A to 1B.