PURPOSE: To ensure the bonding of each core and permit the same to have a sufficiently large strength, by bonding a square pillar-shaped core and a rectangular core together with a bonding agent, applying a bonding pressure to the bonded cores with a mechanical pressure, and supplying a DC current through a coil wound on the square pillar-shaped core thereby to generate an electromagnetic force for hardening the bonding agent.

CONSTITUTION: A square pillar-shaped core 8 constituted by a laminate of strip iron cores is inserted into a bobbin-like insulator 10 wound with a coil 9 and is secured so that both ends of the core 8 project from the corresponding ends of the insulator 10. The core 8 is disposed between two rectangular cores 11, 12 having a required crosssectional area and is secured by means of a bonding agent 13W16. The assembly is mechanically pressed by means of pressing jigs 17, 18, and a DC current is supplied to the coil 9 from a power source 21 thereby to produce an electromagnetic force. The DC current is supplied to the coil 9 until the bonding agent 13W16 hardens after the mechanical pressure is removed.