A back-up power supply system (1) is configured to supply electric power from an electric storage device (5) to loads (3) when a power supply (2) is defective. The back-up power supply system (1) includes a first voltage conversion circuit (6) configured to convert an output voltage of the electric storage device (5). The loads (3) include a first load (31) and a second load (32). The back-up power supply system (1) is configured to supply power from the electric storage device (5) to the first load (31) not via the voltage conversion circuit (6), and to supply electric power from the electric storage device (5) to the second load (32) via the voltage conversion circuit (6).