To provide a slim generator capable of obtaining a constant output, facilitating system linkage, preventing the effects of power failure from spreading as a cogeneration auxiliary device, if the power failure occurs, eliminating polarity switching at the time of reversible rotation, and simplifying power factor adjustment without requiring an expensive breaker.

A brush 8 which contacts a commutator 7 and a stator core 5 are configured to be circumferentially moveable relative to the commutator 7, without distinguishing a stopping state of a rotor 6 from its driving state; commutator segments 71 forming a commutator 7 are tubularly arranged, while imparting skew in the axial direction; the brush 8 is arranged so as to move relative to a rotor 6 along the axial direction, while touching the commutator 7; a field winding 1 is connected with relative AC power supplies through a change-over (first) switch 10; and the brush 8 is connected with one of the AC power supplies via a second switch 11 interlocking with the connection destination of an output terminal and the changeover (first) switch 10 at all times.