To provide a humidistat which prevents the adsorption of water in a preheating area and is excellent in moistening efficiency and dehumidifying efficiency.

A moisture absorption rotor 8, a moisture absorption air passage 13 which makes water contained in air be adsorbed by the rotor 8 and discharges dehumidified air, a regeneration air passage 14 which makes water be desorbed from the rotor 8 by hot air and discharges humidified air, and a preheating passage 15 which is formed downstream in the rotational direction of the rotor 8 from the passage 14 in the rotor 8, recovers heat from the heated rotor 8, and supplies preheated air to the passage 14 are provided. An air introduction port 20 is formed downstream from the rotor 8 in the air supply direction of a preheating passage 15.