An automotive electric power supply system comprising - an alternator that is driven by an engine to generate alternating-current electric power; - a rectifier that is adapted to rectify the alternating-current electric power that is generated by the alternator to output direct-current electric power; characterized by comprising - a first DC/DC converter that is connected by means of alternator wiring to a higher-voltage output terminal of the rectifier, and that is adapted to convert a voltage value of an output voltage of the rectifier to output a different direct-current voltage; - a first electricity storage apparatus that is connected by means of load wiring to the first DC/DC converter, and that is adapted to supply electric power to a load; - a second DC/DC converter that is connected by means of alternator wiring to a higher-voltage output terminal of the rectifier, and that is adapted to convert a voltage value of an output voltage of the rectifier to output a different direct-current voltage; - a second electricity storage apparatus that has an electricity storage capacity that is less than the first electricity storage apparatus, and that is connected to the second DC/DC converter; - a regulator circuit that is connected to the alternator wiring or the load wiring, and that is adapted to supply a field current to a field winding of the alternator; and - a controlling circuit that is adapted to control driving of the first DC/DC converter and the second DC/DC converter to store the generated electric power from the alternator in the first electricity storage apparatus and the second electricity storage apparatus, and - a rotation sensor that is adapted to detect rotational speed of the engine or a rotor of the alternator, wherein either the first DC/DC converter or the second DC/DC converter is constituted by a DC/DC converter (100, 102) that has a voltage conversion ratio that is approximately 1/n, where n is an integer; and wherein the controlling circuit is adapted to change n of the voltage conversion ratio of the DC/DC converter (100, 102) in response to the rotational speed that is detected by the rotation sensor, and perform feedback control of the field current so that an output of the DC/DC converter (100, 102), of which n of the voltage conversion is changed, becomes a target output voltage of the DC/DC converter (100, 102). - an alternator that is driven by an engine to generate alternating-current electric power; - a rectifier that is adapted to rectify the alternating-current electric power that is generated by the alternator to output direct-current electric power; - a first DC/DC converter that is connected by means of alternator wiring to a higher-voltage output terminal of the rectifier, and that is adapted to convert a voltage value of an output voltage of the rectifier to output a different direct-current voltage; - a first electricity storage apparatus that is connected by means of load wiring to the first DC/DC converter, and that is adapted to supply electric power to a load; - a second DC/DC converter that is connected by means of alternator wiring to a higher-voltage output terminal of the rectifier, and that is adapted to convert a voltage value of an output voltage of the rectifier to output a different direct-current voltage; - a second electricity storage apparatus that has an electricity storage capacity that is less than the first electricity storage apparatus, and that is connected to the second DC/DC converter; - a regulator circuit that is connected to the alternator wiring or the load wiring, and that is adapted to supply a field current to a field winding of the alternator; and - a controlling circuit that is adapted to control driving of the first DC/DC converter and the second DC/DC converter to store the generated electric power from the alternator in the first electricity storage apparatus and the second electricity storage apparatus, and - a rotation sensor that is adapted to detect rotational speed of the engine or a rotor of the alternator,