A method of estimating, as the charging rate of an electricity storage system, the internal condition of the electricity storage system including plural batteries, and an internal condition estimating system utilizing that method are provided. A voltage measuring unit 3 measures voltages of all batteries 1 configuring an electricity storage system block 2. A system cell voltage determining unit 5 determines a system cell voltage that will be a reference for an estimation of the SOC value of the electricity storage system block 2 based on the voltage value of the plural batteries 1 measured by the voltage measuring unit 3. The system cell voltage determining unit 5 obtains the system cell voltage by giving a weighting to the voltage value in accordance with the pre-calculated SOC value of the electricity storage system block 2. The system SOC estimating unit 6 estimates the SOC value of the electricity storage system block 2 based on the current flowing through the battery 1 and the system cell voltage.