PURPOSE: To perform delicate temperature and humidity controls by a method wherein a compressor operating time is determined unconditionally on the base of a temperature gradient during the stoppage of the compressor immediately before the operation of the compressor and when the room temperature reaches a thermo-ON point or a thermo-OFF point, the compressor is operated or stopped.

CONSTITUTION: An initial operation time t1 of the compressor 8 is determined unconditionally by a microcomputer 9 on the base of the temperature gradient during the first stoppage of the compressor and a second operation time t2 of the compressor is determined unconditionally on the base of the temperature gradient during the second stoppage of the compressor. When the room temperature reaches the thermo-ON point during the fourth stoppage of the compressor 8, an electromagnetic switch 8a is closed by the microcomputer 9 and the compressor 8 is operated to lower the room temperature down to the thermo-OFF point. Thus, when the thermo-OFF point is reached, the temperature control is made again by the microcomputer 9 on the base of the temperature gradient. Therefore, if the compressor stopping time is set short, it is possible to control the variation of the room temperature below the difference between the thermo- ON point and the thermo-OFF point.