A humidity control device, comprising: a casing provided with a first humidity control chamber, a second humidity control chamber, and a bypass passage; a refrigerant circuit including a compressor, and a first adsorption heat exchanger and a second adsorption heat exchanger which are respectively provided in the first and second humidity control chambers (37, 38), the refrigerant circuit being capable of switching each of the first and second adsorption heat exchangers (51, 52) to a condenser or an evaporator; a flow path switching mechanism which switches an air flow passage in the casing; a controlling unit which controls the refrigerant circuit and the flow path switching mechanism so as to perform a first humidity control operation or a second humidity control operation, wherein in the first humidity control operation, the bypass passage is closed, and outside air taken into the casing is supplied into a room through one of the first and second adsorption heat exchangers (51, 52), and room air taken into the casing is exhausted to outside through the other of the first and second adsorption heat exchangers (51, 52); and in the second humidity control operation, the bypass passage is opened, and the outside air taken into the casing is supplied into the room through one of the first and second adsorption heat exchangers (51, 52) and the bypass passage, and the room air taken into the casing is exhausted to the outside through the other of the first and second adsorption heat exchangers (51, 52), characterised in that, if a target operating capacity (Ctg) of the compressor falls below a predetermined lower limit operating capacity while the first humidity control operation is performed, the controlling unit controls the refrigerant circuit and the flow path switching mechanism to switch from the first humidity control operation to the second humidity control operation. a casing provided with a first humidity control chamber, a second humidity control chamber, and a bypass passage; a refrigerant circuit including a compressor, and a first adsorption heat exchanger and a second adsorption heat exchanger which are respectively provided in the first and second humidity control chambers (37, 38), the refrigerant circuit being capable of switching each of the first and second adsorption heat exchangers (51, 52) to a condenser or an evaporator; a flow path switching mechanism which switches an air flow passage in the casing; a controlling unit which controls the refrigerant circuit and the flow path switching mechanism so as to perform a first humidity control operation or a second humidity control operation, wherein in the first humidity control operation, the bypass passage is closed, and outside air taken into the casing is supplied into a room through one of the first and second adsorption heat exchangers (51, 52), and room air taken into the casing is exhausted to outside through the other of the first and second adsorption heat exchangers (51, 52); and in the second humidity control operation, the bypass passage is opened, and the outside air taken into the casing is supplied into the room through one of the first and second adsorption heat exchangers (51, 52) and the bypass passage, and the room air taken into the casing is exhausted to the outside through the other of the first and second adsorption heat exchangers (51, 52), characterised in that, if a target operating capacity (Ctg) of the compressor falls below a predetermined lower limit operating capacity while the first humidity control operation is performed, the controlling unit controls the refrigerant circuit and the flow path switching mechanism to switch from the first humidity control operation to the second humidity control operation.