A power supply system which includes a first system, a second system, and a system-to-system switch. The first system delivers electrical power from a first power supply to an electrical load through a first power feeding path. The second system delivers electrical power from a second power supply to the electrical load through a second power feeding path. The system-to-system switch is arranged in a connecting path which connects the first and second power feeding paths together. The power supply system also has an in-system switch which is disposed in the second power feeding path between a junction of the second power feeding path and the connecting path and the second power supply. When it is determined that a malfunction has occurred in the second system in a condition where the system-to-system switch and the in-system switch are closed, the system-to-system switch is opened. The power supply system controls an opening-closing operation of the in-system switch when the system-to-system switch is opened, thereby limiting flow of electrical current from the second power supply to the second system and prevents voltages applied to the system-to-system switch from undesirably dropping due to a surge voltage developed in the second system which arise from the limitation of flow of the electrical current.