Abstract: The present invention provides a dew point temperature sensor, detects a dew point temperature of supply air that is supplied to a dry room (i.e., the dew point temperature of process side dried air cooled by a cold water coil), and supplies such to a control apparatus as a supply air dew point temperature tdpv. When the supply air dew point temperature tdpv decreases, the control apparatus lowers the rotational speed of a regeneration side fan and a motor (i.e., a motor that drives a desiccant rotor). Furthermore, the dew point temperature of return air and the like may be detected instead of the dew point temperature of the supply air. In addition, the rotational speed of the regeneration side fan alone may be lowered. In addition, the present invention may be of a type that does not comprise the cold water coil or of a type wherein the return air does not return to the process side air.