PURPOSE: To reduce an electric resistance of a hexagonal ferrite ferromagnetic powder without damaging a magnetic characteristic by a method wherein a cobalt ferrite film is formed on the surface of the hexagonal ferrite ferromagnetic powder.

CONSTITUTION: In order to form a cobalt ferrite film on the surface of a hexagonal ferrite ferromagnetic powder, the hexagonal ferrite ferromagnetic powder is dispersed in an aqueous solution containing a cobalt salt and a ferrous salt and these are heated at a temperature of 60°C or higher for five hours or longer. An amount of the cobalt ferrite film is set at a desired amount by changing an amount to be used of the cobalt salt and the ferrous salt and an amount to be added of the hexagonal ferrite ferromagnetic powder. When the amount of the cobalt ferrite film is 0.1weight% or lower, an electric resistance of the ferromagnetic powder cannot be lowered down to a prescribed value; on the other hand, when it is 15weight% of higher, magnetic characteristic of the ferromagnetic powder is lowered which is not favorable.