PURPOSE: To heat-treat the threaded part of an object to be heat-treated uniformly in a short time by forming a high-frequency electric circuit which heats said threaded part by high-frequency current along the axial direction of the threaded part and flowing secondary induction current along the axial direction of the threaded part.

CONSTITUTION: A heating coil 12 of a closed circuit is formed around an object 10 to be heat-treated by a pair of linear main conductors 131, 132 and a pair of loop-shaped subconductors 141, 142. A threaded part 11 is disposed coaxially to the coil 12, and high-frequency induction current 18 is flowed from a power source 16 to the coil 12. The secondary induction current 19 flows in the direction opposite from that of the current of the main conductors 131, 132 in a manner as to orthogonally intersect screw crest parts 20 and screw trough parts 21, thereby forming a closed circuit. The object 10 is rotated relatively in the arrowed direction 22 around the axis. Since the width I of the main conductors 131, 132 is small, no chevelure phenomenon occurs. In the subconductors 141, 142, the high-frequency induction currents 181, 182 offset each other. Thereby, the threaded part 11 may be uniformly heated.