In a brush holder accommodating part 26d of a gear housing 26, paired flat surface parts 26c and paired curved parts 26b are alternately disposed so as to be formed into an elliptical shape, one of the paired flat surface parts is formed with first heat sinks 26i, and two brushes 20 and 20 mounted on a brush holder unit 19 accommodated in the brush holder accommodating part 26d are disposed near the first heat sinks 26i.