A power storage device has a NaS battery for storing electric power, a Hall current detector for measuring charge/discharge current value I of the NaS battery, a bidirectional converter for converting electric power between a direct current and an alternating current bidirectionally, and a controller for controlling the power storage device. The controller integrates the charge/discharge current value I of the NaS battery and calculates the calculated value of the discharge capacity in the NaS battery. The controller calculates estimated error Er(t2) of the calculated value of the discharge capacity, specifies NaS battery 1004 that requires the correction of the calculated value of the discharge capacity, charges/discharges the NaS battery to a depth of discharge at which the calculated value of the discharge capacity is corrected, and corrects the calculated value of the discharge capacity in the charged/discharged NaS battery.