PURPOSE: To enable a molded rectangular winding iron core to be kept unchanged in shape till an annealing process is finished by a method wherein an L-shaped metal piece is fastened to each of the outer surfaces of the corners of the molded rectangular winding iron core.

CONSTITUTION: A molding piece 13 is inserted into a circular iron core element body to mold it into a rectangular winding iron core B, an L-shaped metal piece 19 is provided and fastened to each of the outer surfaces of the corners of the molded rectangular winding iron core B by a fastening band 20, and the leg and yoke of the winding iron core B are set nearly rectilinear by pressing the L-shaped metal pieces 19 against them, whereby the winding iron core B is kept well in a rectangular shape.