PURPOSE: To save electric power, and to obtain a uniform heating temperature, by pre-heating the whole roll by induction heating, and after that, soaking it by heating of an electric furnace.

CONSTITUTION: A roll of a cold material is inserted into a heating furnace which has combined and electric furnace heating method and an induction heating method, and is induction-heated by an electric power supply of 50W1,000Hz frequency. Subsequently, at the time point when the roll has reached an object heating temperature, the roll is set to a uniform heating temperature by switching to the electric furnace heating method. In this way, electric power is saved, and a uniform heating temperature is obtained.