PURPOSE: To effectively utilize an additional heat source by computing a mean drying of grains during drying so that voltage for energizing a heater for additionally heating dehumidified air produced by a dehumidifier is controlled at the mean drying thus computed.

CONSTITUTION: During a dehumidifying operation, moisture of grains is detected by a moisture detector 3 to compute a reduction rate by drying, whereby a heater 4 of a dehumidifier 2 is controlled based on the reduction rate thus computed and outdoor temperatures detected by an outdoor temperature sensor 39. For example, when one heater 4 is provided, and the computed mean drying is low and the detected outdoor temperature is low, the voltage fed to the heater 4 is high, whereas when the mean drying is high and the detected temperature is high, low voltage is applied to the heater 4 to additionally heat the dehumidified air from the dehumidifier 2, so that the grains are dried by the dehumidified air thus heated.