To prevent useless consumption of energy by changing the feed rate of dry air in accordance with the water content of a raw material of plastics in the inside of a drying treatment cylinder.

The drying device for a raw material of plastics is equipped with an outside air-dehumidifying means 1 for introducing outside air and dehumidifying it, a heating means 6 for heating the dehumidified air sent from the outside air-dehumidifying means and a heated air-circulating means 15 which feeds heated air sent from the heating means into a drying treatment cylinder housing the raw material of plastics and again returns a part of hygroscopic high temperature air discharged from the drying treatment cylinder. The above-mentioned drying device for the raw material of plastics is equipped with a humidity detection means 17 detecting humidity of discharged air, a flow rate-regulating means 16 provided in a supply path of dehumified air and a controlling means 18 which controls the flow rate regulating means so that flow rate is increased when humidity is high and flow rate is reduced when humidity is low on the basis of a signal outputted from the humidity detetion means.