To maintain the cooling efficiency of an impeder core, and to improve the rigidity and the mechanical strength of an impeder reducing the occupied cross sectional area.

An electroseamed tube P is manufactured by rounding a traveling plate bar T in a tubular shape, and pressing and welding both ends thereof by a squeeze roll 8 while heating them by an induction heating coil 6. A ferrite core 21 of substantially arc-shaped section which is located immediately below the induction heating coil 6 to increase the heating efficiency is provided on an upper surface part 25 of an impeder body 24 of an impeder 22 to be inserted in the tube P. A protective layer 29 formed of a fluoor resin provided on the upper surface part 25 of the impeder body 24 to receive spatter c during the welding. A cooling water feed pipe 27 is embedded at a tip side thereof in a recessed groove 26 of the impeder body to cool the ferrite core through a passage 21a between the ferrite core and a flat part.