A power storage system includes a power storage device and a controller. The power storage device includes a plurality of power storage stacks connected in series. The controller controls a discharge processing based on voltage values of power storage elements in each power storage stack. The controller determines an abnormal state according to a voltage fluctuation after the voltage values are made uniform. The controller performs a first processing when the following conditions i) and ii) are satisfied and performs a second processing when the following conditions i) and iii) are satisfied. The condition i) is that the power storage elements included in the plurality of power storage stacks are divided into a plurality of groups, each group consists of the power storage elements included in a same power storage stack. The condition ii) is that the voltage values of the power storage elements are different in each group. The condition iii) is that, in the plurality of groups in each of which the voltage values are made uniform, the voltage values are different among the plurality of groups.