To provide a drying device that can optimize the amount of heat to be applied to a material to be dried.

Heat generated by a first heater 6, a second heater 7, a third heater 8 and a fourth heater 9 is applied to a resin material accommodated in a drying hopper 2. Temperatures of the first heater 6, the second heater 7 and the third heater 8 are detected by temperature sensors 11, 12, 13, respectively. Based on the detected temperatures, an accumulated heat value as an accumulated value of the amount of heat retained in the resin material is calculated. Based on the accumulated heat value, heat generation of the first heater 6, the second heater 7, the third heater 8 and the fourth heater 9 is controlled. The mount of heat retained in the resin material is increased from a reference point. After the resin material is kept retaining the amount of heat increased by a predetermined additional amount from the reference point for a predetermined time, a cycle of heat amount change for reducing the amount of heat retained in the resin material is repeated.