A vehicular power supply system comprising: a starter-generator connected to an engine of a vehicle; a main switch controlled to be turned on or off; a starter switch controlled to be turned on or off; wherein the engine can be started either by the starter switch or by kick drive; a battery having a positive electrode connected to one end of the starter switch and a negative electrode connected to a grounding wire; a relay circuit configured to electrically connect a first contact to a second contact connected to the positive electrode when turned on, and configured to interrupt electrical connection between the first contact and the second contact when turned off; a smoothing capacitor having one end connected to an output terminal (TOUT) and the other end connected to the negative electrode of the battery; a three-phase bridge circuit having one end connected to the first contact and to one end of the main switch, having the other end connected to the grounding wire, and configured to perform motor driving for the starter-generator by performing rectification control or phase control for AC power supplied from the starter-generator and outputting DC power from between the one end and the other end or by converting DC power of the battery to AC power and supplying the AC power to the starter-generator; a control circuit connected between the other end of the main switch and the grounding wire, supplied with power from the other end of the main switch, and configured to control the three-phase bridge circuit and the relay circuit; characterized in that it further comprises: a first control MOSFET transistor having the drain connected to the one end of the three-phase bridge circuit and the source connected to the output terminal (TOUT); a first control body diode having a cathode connected to the drain of the first control MOSFET transistor and an anode connected to the source of the first control MOSFET transistor; a second control MOSFET transistor having the drain connected to the output terminal (TOUT) and the source connected to the positive electrode of the battery; a second control body diode having a cathode connected to the drain of the second control MOSFET transistor and an anode connected to the source of the second control MOSFET transistor; and wherein the control circuit is further configured to control the first control MOSFET transistor and the second control MOSFET transistor. a starter-generator connected to an engine of a vehicle; a main switch controlled to be turned on or off; a starter switch controlled to be turned on or off; wherein the engine can be started either by the starter switch or by kick drive; a battery having a positive electrode connected to one end of the starter switch and a negative electrode connected to a grounding wire; a relay circuit configured to electrically connect a first contact to a second contact connected to the positive electrode when turned on, and configured to interrupt electrical connection between the first contact and the second contact when turned off; a smoothing capacitor having one end connected to an output terminal (TOUT) and the other end connected to the negative electrode of the battery; a three-phase bridge circuit having one end connected to the first contact and to one end of the main switch, having the other end connected to the grounding wire, and configured to perform motor driving for the starter-generator by performing rectification control or phase control for AC power supplied from the starter-generator and outputting DC power from between the one end and the other end or by converting DC power of the battery to AC power and supplying the AC power to the starter-generator; a control circuit connected between the other end of the main switch and the grounding wire, supplied with power from the other end of the main switch, and configured to control the three-phase bridge circuit and the relay circuit; characterized in that it further comprises: a first control MOSFET transistor having the drain connected to the one end of the three-phase bridge circuit and the source connected to the output terminal (TOUT); a first control body diode having a cathode connected to the drain of the first control MOSFET transistor and an anode connected to the source of the first control MOSFET transistor; a second control MOSFET transistor having the drain connected to the output terminal (TOUT) and the source connected to the positive electrode of the battery; a second control body diode having a cathode connected to the drain of the second control MOSFET transistor and an anode connected to the source of the second control MOSFET transistor; and wherein the control circuit is further configured to control the first control MOSFET transistor and the second control MOSFET transistor. a first control MOSFET transistor having the drain connected to the one end of the three-phase bridge circuit and the source connected to the output terminal (TOUT); a first control body diode having a cathode connected to the drain of the first control MOSFET transistor and an anode connected to the source of the first control MOSFET transistor; a second control MOSFET transistor having the drain connected to the output terminal (TOUT) and the source connected to the positive electrode of the battery; a second control body diode having a cathode connected to the drain of the second control MOSFET transistor and an anode connected to the source of the second control MOSFET transistor; and wherein the control circuit is further configured to control the first control MOSFET transistor and the second control MOSFET transistor.