To decrease two cores of control wiring from a job site to a controller for each controller by setting a voltage, which is to be applied through a second limit switch and an electroresistor serially connected thereto to an auxiliary relay, into voltage between the recovery voltage of the auxiliary relay and an operating voltage.

A first limit switch LS1a at a contact (a) detects the opening position of a damper. A second limit switch LS2b at a contact (b) detects the closing position of the damper. An auxiliary relay X3 is connected parallel so as to be excited when any one of first and second limit switches LS1 and LS2 is turned on. An electroresistor R serially connected to the contact (b) of the second limit switch LS2b is included and the electroresistance value of this electroresistor R is set so that a voltage value to be applied through the second limit switch LS2 to the auxiliary relay X3 can be greater than the recovery voltage of the auxiliary relay X3 and can be lower than the operating voltage.