PURPOSE: To imporve the environmental comfortableness by a constitution wherein, in case of dehumidification, the temperature difference between suction air and discharge air is compared with the specified value to control a heater by comparing the temperature difference with the preset value.

CONSTITUTION: A suction air temperature A and a dischare air temperature B are input into a calculator 15 to calculate the difference ΔT1 between the two temperature A and B to be input into another calculator 16. The calculator 16 wherein a specified temperature difference C is already input calculates the difference ΔT2 between the temperature ΔT1 and the specified temperature difference C to input into a transistor Tr. When the ΔT1 is lower than the temperature difference C, the transistor Tr is turned ON to supply a heater relay coil 17c with power turning a heater ON while if the ΔT1 is higher than the temperature difference C, the heater is turned OFF. Through these procedures, the difference between suction air temperature and discharge air temperature may be kept constant improving the environmental comfortableness.