To obtain an appropriate drying temperature at all times by connecting a hot air generator and a feeding port at a drier lower part through a hot air blow-in duct and a hot air feeding duct and providing a main exhaust damper and a sub exhaust damper in the hot air blow-in duct.

The hot air generator is mounted adjacently to a drier 10 and a hot air feeding port at the lower part of the drier 10 and the hot air generator are connected by the hot air blow-in duct 13b of the hot air generator and the hot air feeding duct 14. To the hot air blow-in duct 13b, the main exhaust damper 60 and the sub exhaust damper 70 are attached. By respectively adjusting the dampers 60 and 70, the appropriate drying temperature (about 80°C to about 120°C) is obtained. A hot blast feeding temperature from the hot blast generator is variably adjusted by letting hot air go by the main exhaust damper 60, and when the temperature rises too much, outdoor air is supplied and mixed and the temperature is adjusted within the temperature range of about 80°C to about 120°C by the sub exhaust damper 70.