PURPOSE: To obtain various sorts having different lengths by an injection molding method by injecting ferrite powder mixture resin in a mold having a side opening, feeding cooled and solidified sheetlike molded form out of the mold except its end side, then injecting the resin in a molding space, and integrating with a molded form immediately before repeatedly.

CONSTITUTION: A side opening 3 is formed at a mold 1 having a molding space 2 for molding a sheetlike molded form. At the time of first injection molding, the space 2 is closed with a top 4, and ferrite powder mixture resin is injection molded. After the resin is cooled and cured, the top 4 is removed, a molded form S1 in the space 2 is fed at a specific pitch (a) from the opening 3 to an external space, and stopped in a state that an end side of the form S1 remains in the opening 3. Then, ferrite powder mixture resin is injected in the space 2 closed by the end side of the form S1, cured, and integrated with the form S1 immediately before simultaneously upon forming of next molded form S2. Then, this procedure is repeated.