In a power supply system provided with an assembled battery supervisory device which supervises an assembled battery composed of storage elements connected in series with one another, the system using the assembled battery is obtained which makes it possible to supply electric power to the assembled battery supervisory device, with a simple arrangement. The arrangement is such that electric power for the assembled battery supervisory device is obtained from a part of storage elements constituting the assembled battery. The cell balancer is driven based on the average consumption electric current of the assembled battery supervisory device so as to suppress a deviation in the amount of charge between those storage elements which supply electric power to the assembled battery supervisory device, and those storage elements which do not supply electric power to the assembled battery supervisory device, resulting from the average consumption electric current of the assembled battery supervisory device.