An electric power supply system includes first and second fuel cell stacks, a plurality of fuel tanks, a determination unit configured to determine the state of the first fuel cell stack during operation stop of the first and second fuel cell stacks, and a purging execution unit configured to execute purging by activating the first and second fuel cell stacks according to a determination result and opening on-off valves of the plurality of fuel tanks to supply fuel to the first and second fuel cell stacks.