A power storage apparatus is installed in connection with a power generation system using natural energy generation power. The power storage apparatus is comprised of a control unit and a plurality of electricity storage devices. The control unit is structured to calculate a target value of power station output, to calculate a charge and discharge power instruction of each of the plurality of electricity storage devices, and to instruct the charge and discharge. The control unit calculates a correction value of the charge and discharge power to be charged and discharged by the power storage apparatus to permit the charge rate of the plurality of secondary batteries to follow an individual charge rate target value.