Parameters are set in accordance with a running condition of a vehicle and an environmental condition based on a wiper operation signal of the vehicle. The parameters are used to calculate a stable vehicle interior surface temperature Tna of a window glass. The stable vehicle interior surface temperature Tna and a vehicle interior surface temperature Tn1 calculated at previous sampling are used to calculate a vehicle interior surface temperature Tn in a transient state considering response delay time .tau. of temperature change. A control temperature, at which no dew is formed on the window glass, is determined in accordance with the obtained vehicle interior surface temperature Tn to perform air conditioning control.