PURPOSE: To make the temperature of the metallic material uniform, by selecting the frequency of the electric power source so as to restrict the ratio T/σ, in which T is for the depth of the insufficiently heated part and δ is for the depth of penetration of the induced electric current, within a specified range.

CONSTITUTION: When the thick-walled metallic material held on the skid rail is heated in the heating furnace, an insufficiently heated part generates in the metallic material. Hereupon, if the frequency of the electric power source is selected so as to satisfy a relation 0.1≤T/δ≤0.8 in which T is for the depth of the insufficiently heated part and δ is for the depth of penetration of the induced electric current, the temperature of the metallic material can be uniformized. In case of the metallic material of copper, if the frequency of the electric power source is selected from the range of frequency 1.2W4,400Hz, the electric power for heating the point T equivalent to thickness of the skid mark becomes 10W80% of that for heating the skin; hereby, the ideal heating can be attained.