PURPOSE: To prevent occurrence of level difference due to spring out of commutator piece by making grooves in the opposite side faces of the commutator piece in the axial direction and disposing insulators corresponding therewith.

CONSTITUTION: A commutator segment 1a is made of copper alloy and grooves 6 are made at the leg thereof and in the opposite end faces in the axial direction where insulators, i.e., circular ceramics 7, corresponding with the grooves 6 are provided. Since the insulators 7 are fit in the grooves 6 in the opposite end faces in the axial direction, resistance against removal of the commutator segment can be enhanced. According to the constitution, occurrence of level difference due to spring out of the commutator segment can be prevented resulting in smooth commutation.