A vehicle power supply system, comprising: a control switch that is turned on/off by a user; a starter switch that is turned on/off by the user; a relay circuit that is turned on by energizing a coil having a first end connected to a first end of the control switch, thereby making an electrical connection between a battery connecting terminal connected to a battery and a first end of the starter switch and a first end of a first load (Load1), and is turned off by deenergizing the coil so as to interrupt the battery connecting terminal from the first end of the starter switch and the first end of the first load (Load1); and an engine control unit that controls power supplied from the battery and a motor generator, wherein the engine control unit comprises: a three-phase bridge circuit that has a first end connected to the first end of the first load (Load1) and a second end connected to a ground, the three-phase bridge circuit performing commutation control on an alternating-current power supplied from the motor generator or driving the motor generator; a DC-DC converter connected between the first end of the three-phase bridge circuit and a second end of the control switch; a diode that has an anode connected to the battery connecting terminal and a cathode connected to the second end of the control switch; a voltage detecting circuit that detects a voltage of the battery connecting terminal; a first switch element connected between a second end of the coil and the ground; a second switch element (SW2a, SW2b, SW2c) connected between a second end of the first load (Load1) and the ground; and a control circuit that is supplied with a direct-current power supplied to the first end of the control switch and controls the three-phase bridge circuit and the first and second switch elements (SW1, SW2a, SW2b, SW2c). a control switch that is turned on/off by a user; a starter switch that is turned on/off by the user; a relay circuit that is turned on by energizing a coil having a first end connected to a first end of the control switch, thereby making an electrical connection between a battery connecting terminal connected to a battery and a first end of the starter switch and a first end of a first load (Load1), and is turned off by deenergizing the coil so as to interrupt the battery connecting terminal from the first end of the starter switch and the first end of the first load (Load1); and an engine control unit that controls power supplied from the battery and a motor generator, wherein the engine control unit comprises: a three-phase bridge circuit that has a first end connected to the first end of the first load (Load1) and a second end connected to a ground, the three-phase bridge circuit performing commutation control on an alternating-current power supplied from the motor generator or driving the motor generator; a DC-DC converter connected between the first end of the three-phase bridge circuit and a second end of the control switch; a diode that has an anode connected to the battery connecting terminal and a cathode connected to the second end of the control switch; a voltage detecting circuit that detects a voltage of the battery connecting terminal; a first switch element connected between a second end of the coil and the ground; a second switch element (SW2a, SW2b, SW2c) connected between a second end of the first load (Load1) and the ground; and a control circuit that is supplied with a direct-current power supplied to the first end of the control switch and controls the three-phase bridge circuit and the first and second switch elements (SW1, SW2a, SW2b, SW2c). a three-phase bridge circuit that has a first end connected to the first end of the first load (Load1) and a second end connected to a ground, the three-phase bridge circuit performing commutation control on an alternating-current power supplied from the motor generator or driving the motor generator; a DC-DC converter connected between the first end of the three-phase bridge circuit and a second end of the control switch; a diode that has an anode connected to the battery connecting terminal and a cathode connected to the second end of the control switch; a voltage detecting circuit that detects a voltage of the battery connecting terminal; a first switch element connected between a second end of the coil and the ground; a second switch element (SW2a, SW2b, SW2c) connected between a second end of the first load (Load1) and the ground; and a control circuit that is supplied with a direct-current power supplied to the first end of the control switch and controls the three-phase bridge circuit and the first and second switch elements (SW1, SW2a, SW2b, SW2c).