A first heat exchanger, a desiccant block, and a second heat exchanger are arranged in series. In a dehumidifying operation, a first operation mode and a second operation mode are alternately repeated. In the first operation mode, the first heat exchanger is operated as a condensor or a radiator and the second heat exchanger is operated as an evaporator. In the second operation mode, the first heat exchanger is operated as the evaporator and the second heat exchanger is operated as the condensor or radiator. A pressure reducing amount in the first operation mode is controlled to be smaller than in the second operation mode so that a degree of superheat in the evaporator (second heat exchanger in the first operation mode, first heat exchanger in the second operation mode) in each of the first operation mode and the second operation mode is appropriately controlled to increase a dehumidifying amount.