A deteriorated capacity calculator calculates a deteriorated capacity that is a capacity reduced from the capacity of a cell in an initial state. A gas amount calculator calculates an amount of gas contained in a container for the cell, from the deteriorated capacity. A pressure calculator calculates a pressure inside the container for the cell from the amount of the gas, a volume of a void space of the container for the cell and the temperature of the cell. A pressure monitor outputs a control signal for stopping charging or discharging of the battery when the pressure inside the container for at least any one cell is equal to or greater than a threshold pressure.