PURPOSE: To enable to improve a power factor and heating efficiency while uniformly heating a workpiece without making a coil structure complicated, in heating a shaft or tubular workpiece with an induction heating coil having the same diameter over its entire length, by introducing an electric current passing through the coil to the workpiece.

CONSTITUTION: A workpiece 6 made of a metallic material comprising a shaft shape of irregular diameter having a large-diameter part 61 at the center and small- diameter parts 62, 63 at both ends is heated with an induction heating coil 5 having the same diameter over its entire length. For this purpose, one and the other ends of the workpiece 6 are connected through contacts 7a, 7b to one end of the coil 5 and the terminal 3b of an AC power source 1, respectively. Thus, the workpiece 6 is subject to conjunct heating, i.e. induction heating caused by an induced current and resistance heating caused by a current passing through the workpiece itself between the contacts 7a and 7b. The action of the induction heating is large at the large-diameter part 61, while that of the resistance heating is large at the small-diameter parts 62, 63. Hence, the temp-rising property of the workpiece 6 is compensated by both of them, so that the temp-rising rate can be made uniform over the entire length of the workpiece 6.