PURPOSE: To switch warm-up control to steady control at optimum timing without using a water temperature sensor.

CONSTITUTION: During warm-up control wherein lower portion blowout temperature is considerably low as compared to target blowout temperature, the quantity of air blown by a blower is set to a quantity V1 of air suitable for the warm-up control. When the difference between the target blowout temperature and the lower portion blowout temperature is reduced and a quantity V2 of air suitable for steady control becomes smaller than the quantity V1 of air suitable for the warm-up control, the quantity of air blown by the blower is set to V2. The quantities V1, V2 of air are compared with each other and the smaller is selected whereby the warm-up control can be switched to the steady control at optimum timing without using a water temperature sensor.