PURPOSE: To allow hardening of only the necessary part by using semicircular conductors disposed along the joint parts between a crank arm member and journals and pins as coil members for induction heating.

CONSTITUTION: The semicircular conductor parts 2a, 2b of the coil member 2 are disposed along the joint part 1D of the crank arm member 1C from the outer periphery of the pin 1A. A high-frequency power source 3 is connected to the coil member 2 and high-frequency current is passed thereto. The coil member is disposed similarly to the joint 1E of the journal 1B and the crank arm 1C and the high-frequency current is passed thereto. Only the parts along the joint parts 1D, 1E requiring hardening in order to increase the strength of the crank shaft 1 are inductively heated and hardened, by which the bending deformation of the crank shaft 1 is prevented and the electric power for heating is decreased to the required min. extent.