A humidity controller provided with two adsorption elements (81, 82) and alternately repeating first operation for regenerating the second adsorption element (82) while dehumidifying the air by means of the first adsorption element (81), and second operation for regenerating the first adsorption element (81) while dehumidifying the air by means of the second adsorption element (82). Second air being taken into the humidity controller is the mixed air of indoor air and outdoor air. Mixing ratio of indoor air and outdoor air in the second air can be varied in the humidity controller.
