Abstract: A vehicle power supply system includes: a plurality of battery modules each having a plurality of high-voltage batteries; a case for accommodating the plurality of battery modules; and a cooling circuit having a radiator, a cooling pump, a plurality of battery cooling units for cooling the plurality of battery modules, the case being disposed below a floor panel. In the cooling circuit, the plurality of battery module cooling units are disposed in parallel; and a branch portion which is provided to an upstream side of the plurality of battery module cooling units and a merging portion which is provided to a downstream side of the plurality of battery module cooling units are provided inside the case.