PURPOSE: To prevent any hidrance against the running of a car as well as to aim at the prolongation of service life in a battery, by removing the cause of an abnormal drop in the supply voltage of a car-mounting heating apparatus, and making up a switch for this end, while stopping a continuous rating current to a heating unit for warmth.

CONSTITUTION: When voltage in a power source 3 is normal, a Zener diode 10 is conducted, turning on a transistor 11, then a power transistor 4 is as well turned on, and a heating unit 1 for warmth is controlled for its temperature by the on-off action of a thermostat 2'. When voltage in the power source 3 drops abnormally more than the Zener diode 10, this Zener diode 10 is shut off, turning off the transistor 11 and the power transistor 4 too, and a continuous rating current to the heating unit 1 for warmth is stopped. Thus, an abnormal voltage drop detector 5 detects this drop whereby the continuous rating current to the heating unit for warmth is stopped and voltage in the power source 3 can be prevented from going to drop furthermore.