An in-vehicle power supply system includes: a main battery; an upper power supply unit configured to supply power of a 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; and a capacitor. The upper power supply unit, the capacitor, and an input of the step-down conversion unit are connected to the first power supply line. The main battery and an output of the step-down conversion unit are connected to the second power supply line.