In the casing (11) of a humidity controller (10), a first bypass passage (81) is provided along one of side plates facing each other, and a second bypass passage (82) is provided along the other side plate. In the casing (11), a first heat exchanger chamber (37) and a second heat exchanger chamber (38) are arranged next to each other in the left-to-right direction in a space between the two bypass passages (81, 82). A first adsorption heat exchanger (51) is accommodated in the first heat exchanger chamber (37), and a second adsorption heat exchanger (52) is accommodated in the second heat exchanger chamber (38). Adsorbents are carried on the adsorption heat exchangers (51, 52). During the operation in which humidity of the air is not controlled, a first bypass damper (83) and a second bypass damper (84) are opened, and the air flows through the bypass passages (81, 82) to be drawn into a supply fan (26) or a exhaust fan (25). Thus, accumulation of odor substances in the adsorption heat exchangers (51, 52) during the operation in which humidity of the air is not controlled, can be reduced.