PURPOSE:To make the hardening of only the required peripheral surface of a member possible, by a method wherein the heating coil having a heating conductor and a subordinate conductor on the peripheral surface of the member is provided and so constructed that the magnetic flux of both conductors is cancelled with each other on the unhardening peripheral surface of the member. CONSTITUTION:A member W is inserted into the heating coil C having a heating condustor 1 and two pieces of subordinate conductors 2a and 2b and the heating conductor 1 is applied with power from a leed R1 and the subordinate conductors 2a and 2b being applied with power from a lead R2. At the non hardening peripheral surface X part on the peripheral surface of member W, the heating conductor 1 and the subordinate conductors 2a and 2b opposing the X part are arranged in a straight line and the X part is not heated by cancelling each other with the magnetic flux phi+ generated from the current flowing through the heating conductor 1 and the magnetic flux phi- generated from the current flowing through the subordinate conductors 2a and 2b. On the other hand, at the hardening peripheral surface Y part, because the heating conductor 1 is inside the subordinate conductors 2a and 2b, the magnetic flux phi- of heating conductor 1 is not cancelled by the magnetic flux phi+ of subordinate conductors 2a and 2b and the peripheral surface Y of member W is suitably heated.
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