PURPOSE: To improve the comfortability by averaging the outer air temperature periodically then compensating the setting temperature on the basis of said average temperature thereby coping with the long term variation of outer air temperature.

CONSTITUTION: A memory 50 will digitize the temperature detected by an outer air temperature sensor 31 through an analog/digital converter 34 to accumulate reading periodically upon command from a clock 52. An arithmetic circuit 51 will call for the accumulated outer air temperature periodically to operate an average outer air temperature TA' which is fed to a setting temperature correcting circuit 53. Consequently, said circuit 53 will correct the setting temperature T0 to produce T0'. For example, the setting temperature T0' is corrected low for high average outer air temperature TA'. The long term variation of outer air temperature is considered for setting temperature correction resulting in accurate temperature control.