1. Temperature around the heater core is corrected.
2. The estimated airflow volume that passes through the heater core is estimated from the blower motor voltage control value and air mix actuator opening angle.
3. The estimated air temperature that blows from the heater core is estimated from the corrected temperature around the heater core and the estimated airflow volume that passes through the heater core.
4. The target value of the air mix actuator opening angle is calculated from the estimated air temperature that blows from the heater core, corrected temperature around the heater core, and target temperature.
5. The actual opening angle of the air mix actuator is calculated from its target value.
6. The target value of the air mix actuator opening angle is calculated from its potentiometer voltage.
7. The air mix actuator is controlled while comparing each target volume.