An automotive electric power supply system comprising: a non-insulated DC/DC converter that is configured so as to be able to convert voltage bidirectionally between a higher-voltage input/output terminal and a lower-voltage input/output terminal; an automotive main electric power source that is connected to said lower-voltage input/output terminal of said DC/DC converter by means of a first switch; a capacitor that is connected to said higher-voltage input/output terminal of said DC/DC converter, and that has a rated maximum value that is a higher voltage than a rated voltage of said main electric power source; an alternator that is connected to said higher-voltage input/output terminal of said DC/DC converter, and that has a Lundell rotor to which a field coil is mounted, said alternator being driven to rotate by an engine to generate electric power; a permanent magnet that is disposed on said rotor, and that generates a magnetic field that is in a reverse direction to excitation by said field coil inside a claw-shaped magnetic pole portion (40, 44) of said rotor; a regulator circuit that is connected to said higher-voltage input/output terminal of said DC/DC converter, and that supplies electric power to said field coil; a starter motor that starts said engine; and a power supply control circuit that controls opening and closing of said first switch so as to be in an open state when said engine is at rest, and so as to be set to a connected state after said regulator circuit is operated to make said alternator generate electric power to precharge said capacitor after said engine is started, characterized in that said power supply control circuit is configured so as to set said first switch to said connected state, on detecting that a voltage of said precharged capacitor is higher than a voltage of said main electric power source and then operate said DC/DC converter to step up said voltage of said main electric power source to said rated maximum value of said capacitor, and also to operate said regulator circuit to make said alternator generate electric power, and to charge generated electric power from said alternator to said capacitor. a non-insulated DC/DC converter that is configured so as to be able to convert voltage bidirectionally between a higher-voltage input/output terminal and a lower-voltage input/output terminal; an automotive main electric power source that is connected to said lower-voltage input/output terminal of said DC/DC converter by means of a first switch; a capacitor that is connected to said higher-voltage input/output terminal of said DC/DC converter, and that has a rated maximum value that is a higher voltage than a rated voltage of said main electric power source; an alternator that is connected to said higher-voltage input/output terminal of said DC/DC converter, and that has a Lundell rotor to which a field coil is mounted, said alternator being driven to rotate by an engine to generate electric power; a permanent magnet that is disposed on said rotor, and that generates a magnetic field that is in a reverse direction to excitation by said field coil inside a claw-shaped magnetic pole portion (40, 44) of said rotor; a regulator circuit that is connected to said higher-voltage input/output terminal of said DC/DC converter, and that supplies electric power to said field coil; a starter motor that starts said engine; and a power supply control circuit that controls opening and closing of said first switch so as to be in an open state when said engine is at rest, and so as to be set to a connected state after said regulator circuit is operated to make said alternator generate electric power to precharge said capacitor after said engine is started, characterized in that said power supply control circuit is configured so as to set said first switch to said connected state, on detecting that a voltage of said precharged capacitor is higher than a voltage of said main electric power source and then operate said DC/DC converter to step up said voltage of said main electric power source to said rated maximum value of said capacitor, and also to operate said regulator circuit to make said alternator generate electric power, and to charge generated electric power from said alternator to said capacitor.