To increase the regenerative capacity of a desiccant and make it recover its adsorption capacity quickly by retarding the adsorption of moisture in the treated air by the desiccant and by accelerating the increase of temperature of the regenerated air to decrease its relative humidity when the dehumidifying action of the desiccant is inadequate.

A controller 350 is provided for an auxiliary heating means 310 and a damper 370. When the dehumidifying action of a desiccant is inadequate, especially before the start, the controller actuates the auxiliary heating means 310 to heat the regenerated air and by adjusting the opening of the damper 370 controls the operation in such a manner as to decrease the flow rate of the regenerated air. By thus providing an operating mode preceding a start regenerated air of high temperature with a low relative humidity is produced and a desiccant rotor 103 can be regenerated to recover a good moisture-adsorption capacity. The auxiliary heating means 310 is useful not only in the operation mode of starting step but can also be used as an auxiliary means when the amount of heating of a heat pump is inadequate during operation.