A generator (100) includes a generator winding (103) and an excitation winding (104), and a field winding (102). To converge an output voltage of the generator winding (103) to a target, a field current is varied by increasing/decreasing an energization duty ratio of a switching device (110) connected to the field winding (102). In a duty ratio zero determination unit (2) and a duty ratio zero continuation determination unit (3), when an output duty ratio continues for a predetermined time with duty ratio zero, a duty ratio increase amount restriction unit (4) restricts an upper limit of the duty ratio to a predetermined upper limit when the field current increases. A duty ratio restriction unit (21a) for restricting the duty ratio by a maximum value determined based on a voltage of a smoothing capacitor (113) in place of the determination of the duty ratio being zero may be provided.