An in-vehicle power supply system configured to supply power of a power supply on a vehicle side to an in-vehicle device serving as a load on a vehicle, the in-vehicle power supply system comprising: a main battery configured to charge and discharge the power of the power supply; an upper power supply unit configured to supply the power of the power supply to the main battery; a first power supply line allocated to conduct the power of the power supply of a first voltage; a second power supply line allocated to conduct the power of the power supply of a second voltage lower than the first voltage; a step-down conversion unit configured to step down a voltage of the first power supply line to supply the power of the power supply to the second power supply line; a large electric power load connected to the first power supply line; and a capacitor connected to the first power supply line via a switch unit, wherein the switch unit is configured to control circuit connection between the capacitor and the first power supply line, wherein the upper power supply unit, and an input of the step-down conversion unit are connected to the first power supply line, and wherein the main battery and an output of the step-down conversion unit are connected to the second power supply line; and a control unit configured to control the switch unit, wherein the control unit is configured to, when an ignition of the vehicle is ON, control the switch unit to close to connect a circuit connection of the capacitor to the first power supply line, to charge the capacitor, if the capacitor is not charged; characterized in that the control unit is further configured to, when the ignition of the vehicle is ON: control the switch unit to open to cut off the circuit connection of the capacitor, when charging of the capacitor is completed; and control the switch unit to close to connect the circuit connection of the capacitor, to cause electric charge accumulated in the capacitor to be discharged via the switch unit, in a case in which the power of the power supply is required to be supplied to at least the large electric power load; and the control unit is configured to, when the ignition of the vehicle is OFF: control the switch unit to close to connect the circuit connection of the capacitor, to perform discharge of the capacitor. a main battery configured to charge and discharge the power of the power supply; an upper power supply unit configured to supply the power of the power supply to the main battery; a first power supply line allocated to conduct the power of the power supply of a first voltage; a second power supply line allocated to conduct the power of the power supply of a second voltage lower than the first voltage; a step-down conversion unit configured to step down a voltage of the first power supply line to supply the power of the power supply to the second power supply line; a large electric power load connected to the first power supply line; and a capacitor connected to the first power supply line via a switch unit, wherein the switch unit is configured to control circuit connection between the capacitor and the first power supply line, wherein the upper power supply unit, and an input of the step-down conversion unit are connected to the first power supply line, and wherein the main battery and an output of the step-down conversion unit are connected to the second power supply line; and a control unit configured to control the switch unit, wherein the control unit is configured to, when an ignition of the vehicle is ON, control the switch unit to close to connect a circuit connection of the capacitor to the first power supply line, to charge the capacitor, if the capacitor is not charged; characterized in that the control unit is further configured to, when the ignition of the vehicle is ON: control the switch unit to open to cut off the circuit connection of the capacitor, when charging of the capacitor is completed; and control the switch unit to close to connect the circuit connection of the capacitor, to cause electric charge accumulated in the capacitor to be discharged via the switch unit, in a case in which the power of the power supply is required to be supplied to at least the large electric power load; and control the switch unit to open to cut off the circuit connection of the capacitor, when charging of the capacitor is completed; and control the switch unit to close to connect the circuit connection of the capacitor, to cause electric charge accumulated in the capacitor to be discharged via the switch unit, in a case in which the power of the power supply is required to be supplied to at least the large electric power load; and the control unit is configured to, when the ignition of the vehicle is OFF: control the switch unit to close to connect the circuit connection of the capacitor, to perform discharge of the capacitor.