To solve the problem that, regarding a one turn coil subjecting a metal thin sheet to electromagnetic welding into a seam shape, when seam welding is performed with the conventional coil, the seam width is 3 to 5 mm, and welding with the seam width of ≤1 mm has not been performed.

A large current reciprocating between the current concentration part 1a (width 1 mm) in a coil and the peripheral part 1c is rapidly made to flow, and aluminum sheets 2 plated thereon are welded. For the purpose of preventing the deformation of the current concentration part 1a, forces 5 are applied from the circumference using a fixed plate 3 and an insulting material 4. The aluminum plates 2 are welded at a seam welding width of about 1 mm along the current concentration part 1a.