PURPOSE: To prevent an evaporator from being lowered below a freezing temper ature and suppress a rise of humidity in a vehicle chamber by controlling an internal/outside air switching door when the inlet outside air temperature is low in case of operating a rear seat air conditioner in air conditioning mode.

CONSTITUTION: When an outside air temperature judging means 110 judges that the outside air temperature is less than a determined value, and an air conditioning mode judging means 120 judges that a rear seat air conditioner 3 is operated in air conditioning mode, an internal/outside air switching door 8 is driven on the internal air side when the evaporator temperature is low and on the outside air side when the evaporator temperature is high according to the detection result by an evaporator temperature detecting means 100.