To provide a planar one turn coil for electromagnetic welding as a planar one turn coil for applying electromagnetic welding to thin metal sheets, by eliminating such a defect that the conventional planar coil that, upon welding of aluminum alloy sheets hard to be welded, high power source energy has been required.

A planar one turn coil 3 is connected to a power source 1 and a switch 2. The switch 2 is closed, and pulse large current is made to flow through the coil. Since the width of the central part 4A on which the current is made to flow so as to be concentrated is changed, the aluminum alloy sheets 5A, 5B can be welded at high energy efficiency.