An internal condition estimating system configured to estimate a system SOC of an electricity storage system block comprising a plurality of batteries, the internal condition estimating system comprising: a voltage measuring unit configured to measure voltages of all the batteries of the plurality of batteries; a current measuring unit configured to measure a current flowing through the plurality of batteries; a system cell voltage determining unit configured to determine, based on voltage values of the plurality of batteries measured by the voltage measuring unit, a system cell voltage that is a reference for an estimation of the SOC of the electricity storage system block; and a system SOC estimating unit configured to estimate, based on the current flowing through the battery and the system cell voltage, the SOC of the electricity storage system block, characterized in that the connection of the batteries is a multiple parallel-series connection of plural parallel-connection groups each including plural batteries connected in series, wherein the current measuring unit is installable at one location in the electricity storage system block, and is configured to calculate the current value flowing through the battery that configures the electricity storage system block based on the current value measured at the installation location, the system cell voltage determining unit configured to determine the system cell voltage by giving a weighting to the voltage values in accordance with a pre-calculated SOC of the electricity storage system block, wherein the system cell voltage determining unit comprises: a voltage value memory configured to store voltage values per a time of the plurality of batteries measured by the voltage measuring unit; a voltage value selector configured to select a maximum voltage value and a minimum voltage value among the voltage values stored in the voltage value memory; and a determination condition memory configured to store a weighting condition for the maximum voltage and the minimum voltage. a voltage measuring unit configured to measure voltages of all the batteries of the plurality of batteries; a current measuring unit configured to measure a current flowing through the plurality of batteries; a system cell voltage determining unit configured to determine, based on voltage values of the plurality of batteries measured by the voltage measuring unit, a system cell voltage that is a reference for an estimation of the SOC of the electricity storage system block; and a system SOC estimating unit configured to estimate, based on the current flowing through the battery and the system cell voltage, the SOC of the electricity storage system block, characterized in that the connection of the batteries is a multiple parallel-series connection of plural parallel-connection groups each including plural batteries connected in series, wherein the current measuring unit is installable at one location in the electricity storage system block, and is configured to calculate the current value flowing through the battery that configures the electricity storage system block based on the current value measured at the installation location, the system cell voltage determining unit configured to determine the system cell voltage by giving a weighting to the voltage values in accordance with a pre-calculated SOC of the electricity storage system block, wherein the system cell voltage determining unit comprises: a voltage value memory configured to store voltage values per a time of the plurality of batteries measured by the voltage measuring unit; a voltage value selector configured to select a maximum voltage value and a minimum voltage value among the voltage values stored in the voltage value memory; and a determination condition memory configured to store a weighting condition for the maximum voltage and the minimum voltage.