To provide an energy storage system having high expandability capable of easily increasing and decreasing the number of energy storage devices connected in parallel by constructing the energy storage system in which the plurality of energy storage devices are connected in parallel so that specifications change represented by capacity change is easily performed and by facilitating the management of the plurality of energy storage devices.

In an electric vehicle on which the energy storage system having an energy storage device constituted by connecting a plurality of unit-cells in series and an integrated control device is mounted, the energy storage device, and a contactor and a current sensor are connected in series to the energy storage device. The energy storage device, and the contactor and the current sensor are provided with a control device capable of managing the status of the energy storage device to constitute a unit energy storage module. The plurality of unit energy storage modules are connected in parallel. An electric load to be drive controlled using the electric power of the energy storage device is provided. An integrated control device connected to the electric load and each unit energy storage module is provided.