Each brush holder is formed by bending a holder material made of a metal plate, so that the brush holder is shaped into a generally rectangular tubular body, which includes opposed first and second lateral walls, a top wall and a base wall. A base plate includes holder limiting grooves. Each brush holder is received in a corresponding one of the limiting grooves. The limiting groove limits an increase in a space between the first lateral wall and the second lateral wall of the corresponding brush holder. Furthermore, each brush is received in a corresponding one of the brush holders in a slidable manner in a sliding direction toward and away from a commutator of a motor.