In each of battery modules, a processing circuit manages a battery unit connected to a power line. A communication circuit communicates data to be transmitted or received by the processing circuit. An isolated circuit insulates in a direct current between a first terminal of the communication circuit and a positive-electrode terminal or a negative-electrode terminal of the battery unit, and between a second terminal of the communication circuit and a conductive body to be a common potential of a plurality of the battery modules except for the power line.