An impeder used for an electric resistance tube welding process for welding by heating the opposite edge portions of a strip skelp continuously formed into a tubular or a spirally tubular shape comprises plural laminated metals composed of foils of a metallic magnetic material, said foils being electrically insulated from each other, disposed in an impeder casing as a core. The impeder improves the welding energy efficiency in the electric resistance tube welding process and is excellent in durability.
