A power supply system applied to a vehicle having an idling stop function for executing an automatic stop and an automatic restart on an engine, the power supply system comprising: a power generator; a lithium-ion battery that can be charged with and can discharge generated power generated by the power generator; a lead-acid or lead-free battery that can be charged with and can discharge the generated power; two paths (C1, C2) connecting the lithium-ion battery and the lead-acid or lead-free battery; a switching unit (51, 50) including a relay for switching one of the paths between a conductive condition and a non-conductive condition, and a MOSFET for switching the other path between a conductive condition and a non-conductive condition; an engine restarting unit connected to either the lithium-ion battery side or the lead-acid or lead-free battery side of the switching unit (51, 50) in order to start the engine during the automatic restart; an electrical load of the vehicle, which is connected to the lithium-ion battery side of the switching unit (51, 50); and a control unit (19, 60) configured to implement ON/OFF control on the relay and the MOSFET, wherein the control unit (19, 60) switches both the relay and the MOSFET to the conductive condition while the engine is operative or stopped during the idling stop or a stage after a starting stage of the automatic restart, whereby discharge from the lead-acid or lead-free battery to the electrical load is performed using both of the paths (C1, C2), characterised in that the control unit (19, 60) switches both the relay and the MOSFET to the non-conductive condition during a starting stage of the automatic restart, and wherein the control unit (19, 60) switches the relay to the non-conductive condition for a predetermined time before and after the starting stage of the automatic restart. a power generator; a lithium-ion battery that can be charged with and can discharge generated power generated by the power generator; a lead-acid or lead-free battery that can be charged with and can discharge the generated power; two paths (C1, C2) connecting the lithium-ion battery and the lead-acid or lead-free battery; a switching unit (51, 50) including a relay for switching one of the paths between a conductive condition and a non-conductive condition, and a MOSFET for switching the other path between a conductive condition and a non-conductive condition; an engine restarting unit connected to either the lithium-ion battery side or the lead-acid or lead-free battery side of the switching unit (51, 50) in order to start the engine during the automatic restart; an electrical load of the vehicle, which is connected to the lithium-ion battery side of the switching unit (51, 50); and a control unit (19, 60) configured to implement ON/OFF control on the relay and the MOSFET, wherein the control unit (19, 60) switches both the relay and the MOSFET to the conductive condition while the engine is operative or stopped during the idling stop or a stage after a starting stage of the automatic restart, whereby discharge from the lead-acid or lead-free battery to the electrical load is performed using both of the paths (C1, C2),