PURPOSE: To lightly manipulate an appliance such as a high level damper through a slight force, by using a diaphragm actuator, which employs negative pressure, to drive the appliance.

CONSTITUTION: When only lower blowoff is to be selected, a switch knob 12b is set in a position A so that switch levers 12c, 12d are shifted to positions G, J and one switch lever 12c comes into touch with a fixed contact 12g. As a result, an electrical current from a power source 15 flows to a coil 11b to open a valve 11c so that negative pressure is introduced into a communication port 11g and the chamber 10e of a diaphragm actuator 10 to suck a diaphragm 10a and a rod 10c downward. As a result, a damper 3 is set in a position A' through a lever 7 to close an upper passage 5 and open only a lower passage 6.