An object is to reliably conduct cell balancing operation while suppressing deterioration of batteries and maintaining operating efficiency. When the cell balance control by the cell balancing circuit 3 is started, the system control device 30 sets an on/off device corresponding to the arm where the cell balance control is conducted to the open state; and when the cell balance control is completed, and the difference between the terminal voltage of a battery pack of the arm where the cell balance control has been conducted and the terminal voltages of the battery packs of the other arms falls within a preset allowable range, sets the on/off device, which has been in the open state, to the closed state.