A dehumidifier, comprising: a refrigerant circuit configured to circulate refrigerant and sequentially connecting a compressor, a flow switching device, a first heat exchanger, a pressure reducing unit, and a second heat exchanger to each other by refrigerant pipes; an air passage in which the first heat exchanger, a desiccant material capable of adsorbing and desorbing moisture, and the second heat exchanger are arranged in series; and an air-sending device arranged in the air passage and configured to cause air existing in space to be dehumidified to flow through the air passage, each of the first heat exchanger and the second heat exchanger having a plurality of fins arranged in parallel at an interval from one another so that air passes between the plurality of fins, the interval between fin surfaces of the plurality of fins being set to 1.5 mm to 3.0 mm, the dehumidifier being configured to perform a dehumidifying operation including a first operation mode in which the first heat exchanger is operated as a condensor or a radiator and the second heat exchanger is operated as an evaporator to desorb the moisture retained in the desiccant material, and a second operation mode in which the first heat exchanger is operated as the evaporator and the second heat exchanger is operated as the condensor or the radiator to cause the desiccant material to adsorb moisture from air passing through the air passage, the first operation mode and the second operation mode being alternately switched through switching of a flow passage by the flow switching device. a refrigerant circuit configured to circulate refrigerant and sequentially connecting a compressor, a flow switching device, a first heat exchanger, a pressure reducing unit, and a second heat exchanger to each other by refrigerant pipes; an air passage in which the first heat exchanger, a desiccant material capable of adsorbing and desorbing moisture, and the second heat exchanger are arranged in series; and an air-sending device arranged in the air passage and configured to cause air existing in space to be dehumidified to flow through the air passage, each of the first heat exchanger and the second heat exchanger having a plurality of fins arranged in parallel at an interval from one another so that air passes between the plurality of fins, the interval between fin surfaces of the plurality of fins being set to 1.5 mm to 3.0 mm, the dehumidifier being configured to perform a dehumidifying operation including a first operation mode in which the first heat exchanger is operated as a condensor or a radiator and the second heat exchanger is operated as an evaporator to desorb the moisture retained in the desiccant material, and a second operation mode in which the first heat exchanger is operated as the evaporator and the second heat exchanger is operated as the condensor or the radiator to cause the desiccant material to adsorb moisture from air passing through the air passage, a first operation mode in which the first heat exchanger is operated as a condensor or a radiator and the second heat exchanger is operated as an evaporator to desorb the moisture retained in the desiccant material, and a second operation mode in which the first heat exchanger is operated as the evaporator and the second heat exchanger is operated as the condensor or the radiator to cause the desiccant material to adsorb moisture from air passing through the air passage, the first operation mode and the second operation mode being alternately switched through switching of a flow passage by the flow switching device.