A backup power supply system of the present invention includes a first auxiliary power supply and a second auxiliary power supply capable of supplying electric power to a first load when failure of a main power supply occurs. In the backup power supply system, a second duration is shorter than a first duration. The first duration is the duration until the first auxiliary power supply becomes capable of supplying electric power to the first load. The second duration is the duration until the second auxiliary power supply becomes capable of supplying electric power to the first load.