A battery states estimation method includes the steps of: measuring an electric current value of a battery B at an electric current value detection section and a terminal voltage value of the battery B at voltage detection section at a time of charge/discharge of the battery; and calculating at least one of such states of the battery as a state of charge (SOC), a state of health (SOH) and a full charge capacity based on the electric current value and the terminal voltage value as measured. When a change in the measured electric current value per second is at a predetermined value or higher, at least one of the SOC, the SOH and the full charge capacity of the battery is calculated based on the electric current values and the terminal voltage values measured during a battery charge/discharge period other than the lapsed period.