Abstract: A regulator for a vehicle-mounted alternator is connected to a vehicle-mounted engine control unit through a single signal line. The engine control unit produces serially a plurality of control signals in the form of pulse signals. The regulator includes an external signal receiver circuit, which detects periods of the control signals and duty ratios of the control signals. Control variables of the alternator such as a target voltage and gradual excitation period are determined by the detected periods. Control values of the control variables are determined by the detected duty ratios, respectively. The regulator thus controls a switching device, which supplies excitation current to the alternator based on the determined control variables and the control values.