To drasitically reduce drying time and save energy without increasing temperature of a matter to be dried.

In an early stage of drying wherein the water content is high, heat is absorbed from water in a liquid tank 6 and radiated from a heater 2 to quickly heat air to a temperature higher than that of the matter 10 to be dried so as to enhance evaporation of the water content. The temperature of the matter 10 itself does not rise because heat is consumed as heat of evaporation. After the water content has been decreased, operation is switched to cooling dehumidification, and cooling capacity of a cooler 4 is increased by making a refrigerant flowing out of the heater 2 radiate heat to low temperature water in the tank 6 so that energy is saved.