PURPOSE: To increase a space factor of magnetic metal body of a coiled core by a method wherein a transition layer is provided on the surface of the magnetic metal body and the layer is used as a layer insulation film.

CONSTITUTION: An Ni-Fe system magnetic alloy plate whose thickness is not more than 50μm and which contains 70W90% of Ni, not more than 8% of Mo, not more than 8% of Cu and not more than 3% of Fe or any two of Al, Ti or Si is provided. The plate is heated at 400W900°C in the air, and annealed at 700W1,200°C in an inert gas or N2 gas and an oxide film is formed on the surface of the plate. A wound core with an improved space factor and low cost can be realized by making the core by winding the metal strip thus provided.