PURPOSE: To increase the temp. difference between a gas inlet and a gas outlet by ejecting a charged medium with a rotary disk, throwing up the medium with a high-temp. gas, dropping the medium onto the rotary disk by a dust separator, spraying water into the circulating flow of the medium, and exposing the medium to the gas.

CONSTITUTION: The medium (e.g., small-grain-diameter limestone) is ejected toward the outer periphery by the rotary disk 2 provided at the relatively low position in a casing 1. The medium is injected onto the upper surface of the disk 2 by a medium supply device 3 fixed to the casing. Cooling water is sprayed into the casing from a water nozzle 4. The high-temp. gas from an outlet 5 is blown up from the periphery of the disk 2. The medium dropped from the disk 2 is recovered by a scraper 6. A controlled-humidity gas outlet 7 is provided at a relatively high position of the casing, and the coarse dust in the controlled-humidity gas is recovered by the dust separator and circulated. As a result, a compact gas humidity controller, wherein the temp. difference between the outlet and inlet is increased, can be obtained.