PURPOSE: To improve core plane accuracy without deteriorating the characteristics as a rotary transformer core by injection molding ferrite resin manufactured by mixing specific ferrite powder with thermosetting resin or thermoplastic resin.

CONSTITUTION: Almost sphere ferrite powder (grain diameter = several μm - several 10μm) is mixed with polymer material such as polypropylene and polyamide by 70-98wt.% and ferrite resin, which is the raw material of a core, is manufactured. Then, the ferrite resin is injection-molded and a rotary transformer core is manufactured. Thus, the core plane accuracy is improved without deteriorating the characteristics as the rotary transformer core, the manufacturing time for the rotary transformer core is shortened and the cost is reduced.