PURPOSE: To prevent a motor from damage by providing a motor running duration detecting means and a motor forcedly interrupting means composed of a relay which is actuated in response to the output of the detecting means, so as to forcedly interrupt the running of the motor if it is kept running for a duration longer than necessary.

CONSTITUTION: There are provided both a motor running duration detecting means 10 which detects the running duration of a motor 1 on the basis of the result from the detection of the closed duration of a pressure switch 5, and a motor forcedly interrupting means 10 composed of a relay 11 which is actuated in response to the output of the detecting means 10. In such unusual cases that the motor 1 is kept running for a duration longer than necessary and the pressure in a negative pressure tank (not shown) will not be reduced to a specified value, a capacitor 10d is charged up to a specified voltage under the condition that the pressure switch 5 is kept closed for twenty seconds, to electrify a transistor 10g. And the contact point 11c of a relay 11 of the motor forcedly interrupting means 12 is thereby closed so that a transistor 6c becomes unconductive even if the pressure switch 5 is still remained closed. As a result, the relay 2 becomes inactive to make the motor 1 forcedly interrupted, hence preventing its damage.