There is provided a control device of an alternator in which the alternator is controlled to feedback-control a generated voltage to a target voltage without requiring change of parameters and equations of the control device due to change of the power generation characteristics or electromagnetic specification of the alternator, the electromagnetic specification of the alternator can be handled in a wide range without changing the specification of the control device, and its construction becomes easy. The control device includes an arithmetic portion for comparing a deviation with a predetermined value in response to the deviation of the generated voltage of the alternator from the target voltage, first and second convergence correction portions for converging the generated voltage of the alternator to the target voltage in response to the comparison result of the arithmetic portion, and a search proceeding portion and a search execution portion for searching an ON/OFF duty ratio of applied voltage to the alternator in response to the convergence result of the convergence correction portions and on the basis of a binary search method between a predetermined minimum duty ratio and a predetermined maximum duty ratio.