A state estimation device for estimating a state of an energy storage device, the state estimation device comprising: a controller configured to integrate a current that flows through the energy storage device, which is measured by a current sensor; wherein the controller comprises a memory configured to store a correlation characteristic between a residual capacity C and an open circuit voltage V of the energy storage device, wherein the correlation characteristic includes a constant region, in which the open circuit voltage V is smaller than a predetermined open circuit voltage at a boundary point between the constant region and an other region, in which the open circuit voltage V is larger than the predetermined open circuit voltage at the boundary point, wherein in the constant region a change in the correlation between the residual capacity C and the open circuit voltage V with time is smaller than in the other region of the correlation characteristic,; and wherein the controller is further configured to estimate a full-charge capacity Co of the energy storage device by: when a value of the current is smaller than a current threshold value for a predetermined period of time, measuring, by a voltage detection circuit, an open circuit voltage Vp of the energy storage device at a measuring point P that is included in the constant region, wherein it is determined whether the measuring point P is included in the constant region by comparing the measured open circuit voltage Vp to the predetermined open circuit voltage at the boundary point; calculating a residual capacity Cp of the energy storage device at the measuring point P based on the measured open circuit voltage Vp of the energy storage device at the measuring point P and on the correlation characteristic stored in the memory; and estimating the full-charge capacity Co of the energy storage device based on the calculated residual capacity Cp at the measuring point P and on an accumulated charge-discharge amount X of the energy storage device from a full-charge state to the measuring point P, wherein the full-charge state satisfies a full charge condition. a controller configured to integrate a current that flows through the energy storage device, which is measured by a current sensor; wherein the controller comprises a memory configured to store a correlation characteristic between a residual capacity C and an open circuit voltage V of the energy storage device, wherein the correlation characteristic includes a constant region, in which the open circuit voltage V is smaller than a predetermined open circuit voltage at a boundary point between the constant region and an other region, in which the open circuit voltage V is larger than the predetermined open circuit voltage at the boundary point, wherein in the constant region a change in the correlation between the residual capacity C and the open circuit voltage V with time is smaller than in the other region of the correlation characteristic,; and wherein the controller is further configured to estimate a full-charge capacity Co of the energy storage device by: when a value of the current is smaller than a current threshold value for a predetermined period of time, measuring, by a voltage detection circuit, an open circuit voltage Vp of the energy storage device at a measuring point P that is included in the constant region, wherein it is determined whether the measuring point P is included in the constant region by comparing the measured open circuit voltage Vp to the predetermined open circuit voltage at the boundary point; calculating a residual capacity Cp of the energy storage device at the measuring point P based on the measured open circuit voltage Vp of the energy storage device at the measuring point P and on the correlation characteristic stored in the memory; and estimating the full-charge capacity Co of the energy storage device based on the calculated residual capacity Cp at the measuring point P and on an accumulated charge-discharge amount X of the energy storage device from a full-charge state to the measuring point P, wherein the full-charge state satisfies a full charge condition. when a value of the current is smaller than a current threshold value for a predetermined period of time, measuring, by a voltage detection circuit, an open circuit voltage Vp of the energy storage device at a measuring point P that is included in the constant region, wherein it is determined whether the measuring point P is included in the constant region by comparing the measured open circuit voltage Vp to the predetermined open circuit voltage at the boundary point; calculating a residual capacity Cp of the energy storage device at the measuring point P based on the measured open circuit voltage Vp of the energy storage device at the measuring point P and on the correlation characteristic stored in the memory; and estimating the full-charge capacity Co of the energy storage device based on the calculated residual capacity Cp at the measuring point P and on an accumulated charge-discharge amount X of the energy storage device from a full-charge state to the measuring point P, wherein the full-charge state satisfies a full charge condition.