PURPOSE: To regenerate an adsorption material to perform a dehumidifying operation by a method wherein a less-expensive midnight electrical power is effectively utilized to perform a radiation ceiling cooling, a dehumidifying operation is performed to prevent a dew formation and a waste heat of a heat pump is utilized.

CONSTITUTION: A heat pump A is driven under utilization of less-expensive mid-night electrical power to accumulate cold heat in a cold heat accumulation tank 5. Refrigerant in the cold heat accumulation tank 5 is passed through a refrigerant pipe 20 of a ceiling material 9 at a day-time, thereby cold heat accumulated in the cold heat accumulation tank 5 is utilized so as to perform a cooling operation. Air in a room 14 is circulated in a duct 10, passed through an adsorption material 2 and dried there, the air is taken into the room again to perform a dehumidification of air. In the case that no cooling is carried out, waste heat from the heat pump A is sent to the adsorption material 2 so as to heat the adsorption material 2 and then moisture content in the adsorption material 2 is removed so as to regenerate the adsorption material 2.