A power supply system 1 includes: first power lines (21p, 21n) to which a first battery (B1) is connected; second power lines (31p, 31n) to which a second battery (B2) is connected; a voltage converter (5) which converts voltage; a power converter which converts electric power; a management ECU (71) and converter ECU (73) which operates the voltage converter (5); a smoothing capacitor connected to the first power lines (21p, 21n); and a motor ECU (72) which executes system interruption processing of determining the existence of failure of the contactors (22m, 22s, 32m, 32s) based on a change in voltage of the smoothing capacitor. The management ECU (71) and converter ECU (73) operate the voltage converter (5) so that a state in which the static voltage of the first battery (B1) is higher by at least the determination potential difference than the static voltage of the second battery (B2).