A humidity controller includes a differential pressure detection means (93, 97) for detecting a difference between high voltage and low voltage in a refrigeration cycle of a refrigerant circuit (50) and a control means (30) for controlling the capacity of the compressor (53). The control means (30) reduces the capacity of the compressor (53) when a detected value of the differential pressure detection means (93, 97) exceeds a reduction threshold. Further, the control means (30) stops the compressor (53) when the detected value of the differential pressure detection means (93, 97) exceeds a stop threshold higher than the reduction threshold.