A method for diagnosing failure of a current breaking device 21A included in a power supply system 12 of a vehicle 1 includes: a supply step of supplying power to a first electric load 11 and a first energy storage apparatus 13 by a power supply apparatus 14; a command step of commanding the current breaking device 21A to perform cutoff while power is supplied from the power supply apparatus 14 to the first electric load 11 and the first energy storage apparatus 13; and a determination step of measuring a charge current of a secondary battery 20A by a current sensor 21B while the cutoff is commanded to the current breaking device 21A, and determining presence or absence of failure of the current breaking device 21A based on a measured current value.