PURPOSE: To properly heat works of many kinds and many masses in a mixed flow-single material flow by blowing off compressed air from a nozzle disposed in a furnace against the works when the different mass works are heated in the same heating furnace under the same heating conditions.

CONSTITUTION: In an apparatus in which a work such as a motor rotor is heated and insulating varnish is dropped for impregnation, a ring-shaped heating furnace is constructed with a heat insulating material 4 and a heater 3 and work supporting mechanisms 5 are disposed on the circumference in the heating furnace in an equal interval. In the heating furnace, a brass pipe 1 including blowoff outlets 2 and extending along a shaft center of the work supporting mechanism 5 in a space located on the opposite side to the heater 3, and the brass pipe 1 is supplied with compressed air through a solenoid valve 6. In a process where small, middle, large works W (WSn, WMn, WLn) thrown in randomely are supported by the respective work supporting mechanisms 5 for pitch feeding, the solenoid valve 6 is opened and closed with an output from a sequencer 7 to blow off the compressed air against the works. Hereby, excessive temperature rise is restricted.