A first estimated state of charge is output based on an estimated open voltage of a secondary battery. A second estimated state of charge is output based on the result obtained by integrating the measured current of the secondary battery and the upper limit of the error of the measured current and the capacity of the secondary battery. A third estimated state of charge is output based on the result obtained by integrating the measured current and the lower limit of the error of the measured current and the battery capacity. A fourth estimated state of charge is output based on the result obtained by integrating the measured current and the battery capacity. The fourth estimated state of charge is output as a state of charge within the period of the predetermined period.