PURPOSE: To prevent misoperation in a negative pressure controller of an automobile air conditioning device, by detecting a pressure difference between output negative pressure and negative power source and closing an output port with a solenoid valve if the negative pressure source approaches the atmospheric pressure.

CONSTITUTION: Negative pressure from a negative pressure source acts on an actuator 40 via solenoid valves 1, 10 to control an air mixing damper 50 by a control signal of a temperature control circuit 20. Now if control signals 21, 22 are high even if the actuator 40 reaches an operative limit, pressures of both pressure chambers 34, 35 in a pressure detector 30 are equalized, and a diaphragm 33 is moved downward by a spring 37 to open contacts 21a, 22a, to close valves 3, 13 and reduce waste of power. In this action, the pressure of the negative pressure source approaches the atmospheric pressure due to some cause, if the actuator can not be controlled, similarly a pressure switch 30 is operated to prevent unprepared decrease of control pressure, and misoperation of the air mixing damper is prevented.