A plane commutator includes a plurality of commutator segments made of carbon compound, a resinous boss member having a body portion supporting a base portion of the commutator segments and an outer peripheral portion supporting an outer periphery of the same and a plurality of metal base plates. Each metal base plate has a contact portion in contact with the base portion of the commutator segment, an axial portion extending along a peripheral portion of the boss member and a terminal portion. The axial portion of the metal base plates is disposed on an outer periphery of the plane commutator in parallel with the outer peripheral portions of the boss member to separate the outer peripheral portion from the body portion. The outer peripheral portion moves freely from the body portion as the metal base plates thermally expand.