PURPOSE: To prevent the temperature of blow-out air from being lowered upon introduction of the ambient air under cold climate, by providing an inside air introduction means which is controlled in accordance with a temperature of the ambient air during an ambient air introduction mode, for introducing the inside air into a blower unit by a predetermined amount, in the inlet section of an inside air introduction passage.

CONSTITUTION: An inside and outside change-over manipulating knob is set to an ambient air introduction mode during use of a heater, when a detected temperature of the ambient air is below a predetermined value such as 0 deg. C or the like, a temperature switch 21 is energized to close its contact. Accordingly a solenoid 20 is excited by a battery 20 so that a plunger 20A moves in the direction of the arrow A, and accordingly, an intake door 8 is set to a position indicated by a chain line so as to introduce the inside air by a predetermined volume. Accordingly, the temperature of intake air into a blower unit 1 increases to a value higher than the temperature which is obtained from the ambient air alone, thereby it is possible to enhance the heating efficiency.