To provide an improved flat coil for electromagnetic formation without increasing the thickness of a coil board by obviating the defect of prior art that the flat coil board is so thin that the end part of the coil of slit is easily deformed if high pulse current is impressed.

This flat coil such as a one-turn flat coil is formed of a thin metal board by electromagnetic formation. The deformation of the end part of a slit 4 can be prevented by setting the curvature radius of the end part of the slit 4 of the one-turn flat coil 3 to be more than half (about two times) of the split width, thereby reducing the concentration of current flowing (current density) into the coil.