PURPOSE: To reduce input and prevent humidity increase due to the stop of operation by a method wherein the capacity of a compressor is changed to reduce humidity in a short period of time upon starting the operation with a high dehumidifying capacity and, then, the dehumidifying capacity is reduced and continuous operation is effected with a small capacity.

CONSTITUTION: Upon the starting of operation, the compressor is operated in a high speed or the compressing capacity of the same is increased to operate the same in the condition having high dehumidifying capacity. Then, when a time T has elapsed or a humidity, detected by humidity detection or the like, has arrived at the humidity P in the diagram or lower, the operating speed of the compressor is reduced or the compressing capacity is decreased to reduce the dehumidifying capacity and continuous operation under that condition may be effected. According to this control, increase of humidity due to the stop of operation may be prevented while the input may be reduced by lowering the capacity of the compressor.