The method heats the metal foil made of amorphous soft magnetic material while bringing the metal foil into close contact with a placement surface of a metal base such that the metal foil conforms to the placement surface, to crystallize the amorphous soft magnetic material of the metal foil into nano-crystal soft magnetic material. In the crystallization, the metal foil is heated at a heating temperature to crystallize the amorphous soft magnetic material, the heating temperature being higher than or equal to a crystallization starting temperature at which the amorphous soft magnetic material crystallizes into nano-crystal soft magnetic material and allowing a temperature of the placement surface to be lower than a temperature of the metal foil having temperature rise due to heat generated by self-heating during crystallization, and the heat generated by self-heating of the metal foil during crystallization is absorbed by the base.