PURPOSE: To control cooling operation to be wasteless, by connecting a means of detecting the wet-bulb temperature of intake air, to a control circuit for regulating the operation of an evaporator.

CONSTITUTION: When at least one of a thermoswitch 13 for the wet-bulb temperature of intake air and a thermoswitch 18 for blowoff air is turned off, an electromagnetic clutch 24 is disengaged to stop a compressor 25. A level is set for the wet-bulb temperature on the basis of such a region of the relation between temperature and humidity that the person in the room of automobile feels comfortable. If the set level is 12°C, cooling operation is stopped when the wet-bulb temperature of the intake air is not higher than 12°C. A conventional region A of cooling operation stoppage is based on the dry-bulb temperature. Another region B based on the wet-bulb temperature is added to avoid unnecessary cooling operation. Besides, excessive drying is prevented.