To provide a power supply system for an electric vehicle capable of detecting deterioration of batteries and accurately determining the deteriorat ed state of the batteries.

A capacity deterioration determining means and a battery control part 117 are provided in a power supply system 21 for an electric vehicle provided with the chargeable batteries 102 and battery check device 105 that checks the battery conditions, including the remaining capacity of the batteries. At refresh discharging, the capacity deterioration determining means obtains refreshed capacity of the batteries, based on the sum of discharging capacity at running prior to a refresh discharging and that at refreshing by the refresh discharging. The battery control part 117, that functions as a battery deterioration rate determining means obtains the deterioration rate by comparing the capacity of the batteries with their initial capacity.