Power storage system

A power storage system includes a power storage device including cells connected in parallel with each other; a voltage sensor; a current sensor; a data acquisition unit configured to acquire voltage-current data sets; a linear approximation processing unit configured to perform first-order linear approximation on the acquired voltage-current data sets using a function form, Y=aX+b where Y denotes a voltage between terminals of the power storage device at a time of charging/discharging and X denotes a charge/discharge current, and to obtain constants a, b and a ratio b/a between the constants; and a determination unit configured to determine that variation in SOC has not occurred among the cells when the ratio b/a is within a prescribed constant ratio range, and to determine that the variation in SOC has occurred among the cells when the ratio b/a is not within the prescribed constant ratio range.