An in-vehicle power supply system supplies electric power to a large electric power load and a small electric power load. The in-vehicle power supply system includes a first power supply unit to output electric power having a first voltage higher than a total power supply voltage required by the large and small electric power loads, zone management units to manage predetermined zones on the vehicle, a power supply trunk line unit connecting the first power supply unit and the zone management units and a step-down conversion unit disposed in a zone of one zone management unit of the zone management units and to convert the electric power having the first voltage into electric power having a second voltage lower than the first voltage. The power supply trunk line unit includes a high-voltage power supply line to distribute the electric power having the first voltage.