A vehicle power supply system to be installed in a vehicle, comprising: a battery having a rated voltage lower than a predetermined voltage; a capacitor having a rated voltage higher than the predetermined voltage; a capacitor discharging device that is configured to discharge electric charge stored in the capacitor; and a control device (19a, 24) that is configured to control the capacitor discharging device, characterized in that the control device (19a, 24) is configured to control the capacitor discharging device so that the electric charge stored in the capacitor is discharged and the battery is charged with the discharged electric charge when the vehicle collides or when the capacitor is to be replaced, the battery and the capacitor are electrically connected in series, the capacitor discharging device is connected in parallel to the battery and the capacitor, the capacitor discharging device includes first, second, third and fourth switches connected in series (SW1, SW2, SW3, SW4), one end of the first switch is connected to a positive terminal of the capacitor, one end of the forth switch is connected to the negative terminal of the battery, a connection point between the second and third switches (SW2, SW3) is connected to a connection point between the battery and the capacitor, a fifth switch (SWbatt) is connected to the positive terminal of the battery, a sixth switch (SWcap) is connected the positive terminal of the capacitor, and a charge capacitor is connected between a connection point between the first and second switches (SW1, SW2) and a connection point between the third and fourth switches (SW3, SW4). a battery having a rated voltage lower than a predetermined voltage; a capacitor having a rated voltage higher than the predetermined voltage; a capacitor discharging device that is configured to discharge electric charge stored in the capacitor; and a control device (19a, 24) that is configured to control the capacitor discharging device, characterized in that the control device (19a, 24) is configured to control the capacitor discharging device so that the electric charge stored in the capacitor is discharged and the battery is charged with the discharged electric charge when the vehicle collides or when the capacitor is to be replaced, the battery and the capacitor are electrically connected in series, the capacitor discharging device is connected in parallel to the battery and the capacitor, the capacitor discharging device includes first, second, third and fourth switches connected in series (SW1, SW2, SW3, SW4), one end of the first switch is connected to a positive terminal of the capacitor, one end of the forth switch is connected to the negative terminal of the battery, a connection point between the second and third switches (SW2, SW3) is connected to a connection point between the battery and the capacitor, a fifth switch (SWbatt) is connected to the positive terminal of the battery, a sixth switch (SWcap) is connected the positive terminal of the capacitor, and a charge capacitor is connected between a connection point between the first and second switches (SW1, SW2) and a connection point between the third and fourth switches (SW3, SW4).