PURPOSE: To shorten the high frequency induction heating process of a plurality of ball studs W.

CONSTITUTION: Surfaces to be heated of ball studs W are brought close to and faced with heating coils 11-14 of a shape corresponding to the line of intersection between the plane passing the axis of rotation of the ball studs W and the surfaces to be heated while the ball studs W being moved in the direction of the arrow P. After the movement of the ball studs W is stopped, the high frequency induction current is charged for the specified period of time to a heating coil 10 while the ball studs W being rotated about the rotational axis. Then, the movement of the ball studs W in the direction of the arrow P is restarted, and the surfaces to be heated are brought away from the heating coil 10.