To reduce brush moving force for moving a brush so as to mechanically generate a weak field magnetic state by an electric motor with the brush.

A pair of fans 33A, 33B are fixed on a revolving shaft 23 of an electric motor 21, and a windsail plate 40 is arranged between the fans 33A, 33B. The windsail plate 40 tilts around a spindle 381 on receiving wind pressure accompanying rotations of the fans 33A, 33B. A tilt of a lever 38 is transmitted to a brush 30 through a rotor 44, a slider 43, and a brush holder 45, and the brush 30 moves on a peripheral surface of a rectifier 29 in a direction of the axis 231 of the revolving shaft 23 according to rotating directions of the fans 33A, 33B. A plurality of rectifier segments 291 of the rectifier 29 are so shaped as to move in the peripheral direction of the rectifier 29 toward the direction of the axis (the axis 231 of the revolving shaft 23) of the rectifier 29.