PURPOSE: To save the heat necessary for heating, and to reduce the running cost by providing a humidity detector on the upstream side of an air heater to detect the humidity of the air for air purging, and controlling the air heater.

CONSTITUTION: A humidity detector 7 is provided on the upstream side of an air heater 6, and th signal from the detector 7 is compared in a humidity comparator circuit 8 with a preset reference level. The resulting signal is inputted to a heater controlling circuit 9, and a heater 5 is controlled. In this way when said signal is lower than the set reference level, the heating of the heater in the air heater is stopped.