An apparatus for estimating power storage device electric energy, comprising: a charge/discharge circuit, including electrical pathways, each of the electrical pathways comprising a resistor (R1, R2) and a switch (S1, S2) connected in series with the resistor (R1, R2), one end of the electrical pathways is connected to a positive electrode of the power storage device, while the other end is connected to a negative electrode of the power storage device, the charge/discharge circuit having a resistance value that changes when the switch (S1, S2) switches between the electrical pathways; a voltage detector configured to detect a voltage of the power storage device; a current detector configured to detect a current flowing through the power storage device; a circuit selector configured to select the switch (S1, S2) such that a resistance value of the charge/discharge circuit changes at least once from starting a discharging of the power storage device in a state in which the voltage is equal to or greater than a first threshold value until the voltage becomes less than or equal to a second threshold value, or from starting a charging of the power storage device in a state in which the voltage is less than or equal to a third threshold value until the voltage becomes equal to or greater than a fourth threshold value; an electric charge estimator configured to compute an electric charge by time-integrating the current from a start time of the discharging or the charging to an arbitrarily determined time, and compute a relationship between the electric charge and the voltage; an internal resistance estimator configured to compute an internal resistance of the power storage device, based on the voltages and currents at times when resistance values of the charge/discharge circuit are different since starting the discharging or the charging; and an electric energy estimator configured to compute a relationship between the electric charge and an open voltage of the power storage device based on the relationship between the electric charge and the voltage, the current, and the internal resistance, and to estimate an electric energy of the power storage device based on the relationship between the electric charge and the open voltage, the internal resistance, and a current flowing through the power storage device during discharging or charging. a charge/discharge circuit, including electrical pathways, each of the electrical pathways comprising a resistor (R1, R2) and a switch (S1, S2) connected in series with the resistor (R1, R2), one end of the electrical pathways is connected to a positive electrode of the power storage device, while the other end is connected to a negative electrode of the power storage device, the charge/discharge circuit having a resistance value that changes when the switch (S1, S2) switches between the electrical pathways; a voltage detector configured to detect a voltage of the power storage device; a current detector configured to detect a current flowing through the power storage device; a circuit selector configured to select the switch (S1, S2) such that a resistance value of the charge/discharge circuit changes at least once from starting a discharging of the power storage device in a state in which the voltage is equal to or greater than a first threshold value until the voltage becomes less than or equal to a second threshold value, or from starting a charging of the power storage device in a state in which the voltage is less than or equal to a third threshold value until the voltage becomes equal to or greater than a fourth threshold value; an electric charge estimator configured to compute an electric charge by time-integrating the current from a start time of the discharging or the charging to an arbitrarily determined time, and compute a relationship between the electric charge and the voltage; an internal resistance estimator configured to compute an internal resistance of the power storage device, based on the voltages and currents at times when resistance values of the charge/discharge circuit are different since starting the discharging or the charging; and an electric energy estimator configured to compute a relationship between the electric charge and an open voltage of the power storage device based on the relationship between the electric charge and the voltage, the current, and the internal resistance, and to estimate an electric energy of the power storage device based on the relationship between the electric charge and the open voltage, the internal resistance, and a current flowing through the power storage device during discharging or charging.