A power storage apparatus including a battery and a control circuit controlling charging/discharging of the battery, wherein the control circuit determines, when a predetermined time period that has elapsed after the charging/discharging of the battery was ended is shorter than a polarization elimination time period extending from a time at which the charging/discharging of the battery was ended to a time at which polarization of the battery was judged to have been eliminated, an estimated open-circuit voltage for the battery after the elimination of the polarization of the battery; determines the difference between a first estimated charge state and a charge state determined by a time at which the estimated open-circuit voltage was estimated; and determines a second estimated charge state by summing the state of charge determined by the time at which the estimated open-circuit voltage was estimated and the product of the difference and the reflection coefficient.