PURPOSE:To eliminate the leakage and adherence of resin on the surface of a commutator when casing with the resin by forming a groove for inserting an O- ring on the outer periphery of the commutator which makes contact with a riser which is connected at the terminal of an armature coil. CONSTITUTION:A groove 24 which has suitable width and depth is formed on the outer periphery of a commutator 2 at the part which is connected to a riser 2d. The width of the groove 24 is formed in the size adapted for an O-ring 19 which is engaged with the groove 24, and the depth is formed to be equal to or smaller than the outer diameter of the riser 2d in the diameter of the center of the ring 19 to be engaged. The cut and raised part in which an undercut groove 2e makes contact with the end of the riser 2d is removed due to the groove 24. When a completed assembly is mounted in a casting mold, the ring 19 engaged with the groove 24 of the commutator 2 has no undercut groove 2e at the part which makes contact with the bottom of the groove 24, the neck of the commutator 2 is pressed by the inner surface of the upper mold 11, and resin in a cavity 14 does not leak to the surface of the commutator 2 and flow into the groove 2e.