PURPOSE: To prevent adhesion of insulating resin to the opening of slot and the outer circumferential face of core by fitting a die, having same width as the slot and same number of protrusions as the slots on the inner face, to the outer circumferential face of the core so that the die comes in contact with the outside coil.

CONSTITUTION: A die 4 having protrusions 4a on the inner face, having same width as the slot 2, is fitted onto the outer circumference of a core 1 and insulating resin 5 is filled with a coil 3 being pushed to the inside. A shaft 4 is then inserted below the die 4, and the core 1 burried in the slot 2 is inserted into the die 4 with the coil 3 being connected to a commutator 7. Subsequently, an upper die 8 is placed above the die 4 and then the insulating resin 5 is inserted from above and thermally molded. By such arrangement, adhesion 1 of the resin to the outer circumferential face of core and the opening of the slot is prevented and thereby abrasion of cutter is suppressed and machinability is improved. Furthermore, since the protrusions on the inner face of the die push the outside coil uniformly inward, the coil is positioned evenly and imbalance of armature is suppressed.