In an electric power storage system according to the present invention, in the case of charging, a plurality of capacitors of each circuit block of the electric power storage system are switched to a serial connection to initiate the charging. When the output voltage of power storage means reaches the maximum input voltage of DC-AC conversion means, each capacitor of a number j of circuit blocks is switched to a parallel connection in order of higher block voltage. Also up to the time when the maximum input voltage is reached again, each capacitor of a number j of circuit blocks is switched to a parallel connection in order of higher block voltage. In the case of discharging, pluralities of capacitors of each circuit block of the electric power storage system are switched to a parallel connection to initiate the discharging. When the output voltage of power storage means reaches the minimum input voltage of DC-AC conversion means, each capacitor of a number k of circuit blocks is switched to a serial connection in order of higher block voltage. Also up to the time when the minimum input voltage is reached gain, each capacitor of a number k of circuit blocks is switched to a serial connection in order of higher block voltage. Accordingly, the electric power storage system is hardly affected by the capacitance error of the capacitors, and charging/discharging efficiency can be improved.