PURPOSE: To accurately heat treat a seamed part, by performing the automatical movement of an inductor for induction heating the welded part of a pipe being carried in the step of manufacturing an electric seamed pipe or the like on the basis of a signal from a seam sensor.

CONSTITUTION: A pipe 1a running on supporting-carrying rolls 11 successively passes through a guide roll 2, a welder 3 and inductors 5aW5n. Seam sensors 4, 4a detect the position of a seam 9 along the circumferential direction of the pipe at their positions. In general, the twisting angle θ of the pipe based on the perpendicular upper part is detected and sent to a function generator 8. The generator 8 indicates θ as the quadratic function of x using the position of the roll 2 as an origin and the center of the pipe along the direction of a carrying line as an axis x, to function-indicate the position of the seam 9. This signal 12 is compared with a feedback signal 10 representing the position of the inductor by the position controller 7 of a seam-tracking means 14 to send a driving signal to a persuit driving means 6 on the basis of the resulting deviation. Thus, the inductor 5 is driven by a mechanism 6.