To prevent the conditioned air volume blown into the cabin from failing to reach the target by disposing a hot air bypass passage in the support shaft side of the A/M door, while miniaturizing the heater case.

A hot air bypass passage 16 is disposed in the support shaft side of an A/M door 3 for guiding, in the F/D mode or DEF mode, hot air heated during the air passes a heat exchanger for heating, a heater core for instance, directly to a DEF blowout port 13 with by-passing an air passage 15 having a first air mixing chamber 17. This arrangement enables a large sectional area of the ventilation path in the upstream side of the heat exchanger for heating. Further, a communication port 19 that intercommunicate the air passage 15 and the hot air bypass passage 16 can be opened and closed by means of a face door 23 for opening/closing a face blowout port 12. Accordingly, reduction in the number of part items and miniaturization of a heater unit 1 are achieved.