PURPOSE: To prevent aggravation of riding confortableness due to blowoff of hot air to the inside of a car by bringing a cooler flow passage and a heater flow passage into communication with each other in slipstreams of an evaporator and a heater core and enabling heater core heat to be discharged to the outside of the car.

CONSTITUTION: A cooler passage 2 provided with an evaporator 21, and a heater passage 3 provided with a heater core 31, are connected to a branch point 13 on the rear part of an air intake flow passage 1 provided with an air intake 11 and a blower 12. The branch point 13 is also provided with a flow rate adjustment door 14. Said flow passages 2, 3 communicate with each other in slipstreams of the evaporator 21 and the heater core 31 through a communicating passage 5. Further, a rear heater flow passage 32 behind the communicating passage 5 is opened to the outside of a car from an exhaust port 33, while a branch point 51 of the communicating passage 5 is provided with a flow rate adjustment door 52. Furthermore, the communicating passage 5 and the cooler flow passage 2 are joined with a flow passage 4 which sends air into the car, and a confluence 41 is provided with a flow rate adjustment door 43 which adjusts the mixing ratio.