A hybrid excavator including a lower running body, an upper turning body which is adapted to be turned on the lower running body, a boom having an end rotatably attached to the upper turning body, an arm having an end rotatably attached to the other end of the boom, and a work element rotatably attached to the other end of the arm, the hybrid excavator comprising: an engine disposed in the upper turning body to generate a driving force; a motor generator disposed in the upper turning body to perform an electric power generating operation by the driving force transmitted from the engine; a motor generator control unit disposed in the upper turning body to control operation of the motor generator in response to an externally supplied control signal; an electrical power storage system including a first electrical power storage device disposed in the upper turning body and being capable of storing electricity; a second electrical power storage device disposed in the upper turning body, electrically connected to the motor generator control unit and being capable of storing electricity; characterised by a charge/discharge control unit disposed in the upper turning body to electrically connect the first electrical power storage device and the second electrical power storage device together and controlling a charge/discharge operation between the first electrical power storage device and the second electrical power storage device in response to an externally supplied control signal; a motor (21, 300, 310) disposed in the upper turning body and electrically connected to the second electrical power storage device, the motor (21, 300, 310) being capable of performing a regeneration operation to generate electric energy from mechanical energy and capable of storing the electric energy generated by the regeneration operation in the second electrical power storage device; and a control device disposed in the upper turning body to supply a control signal to at least one of the motor generator control unit and the charge/discharge control unit, wherein the control device is arranged to reduce an electrical power storage target value of at least one of the first electrical power storage device and the second electrical power storage device before the regeneration operation is performed by the motor (21, 300, 310), wherein the electrical power storage target value represents a target voltage. an engine disposed in the upper turning body to generate a driving force; a motor generator disposed in the upper turning body to perform an electric power generating operation by the driving force transmitted from the engine; a motor generator control unit disposed in the upper turning body to control operation of the motor generator in response to an externally supplied control signal; an electrical power storage system including a first electrical power storage device disposed in the upper turning body and being capable of storing electricity; a second electrical power storage device disposed in the upper turning body, electrically connected to the motor generator control unit and being capable of storing electricity; characterised by a first electrical power storage device disposed in the upper turning body and being capable of storing electricity; a second electrical power storage device disposed in the upper turning body, electrically connected to the motor generator control unit and being capable of storing electricity; characterised by a charge/discharge control unit disposed in the upper turning body to electrically connect the first electrical power storage device and the second electrical power storage device together and controlling a charge/discharge operation between the first electrical power storage device and the second electrical power storage device in response to an externally supplied control signal; a motor (21, 300, 310) disposed in the upper turning body and electrically connected to the second electrical power storage device, the motor (21, 300, 310) being capable of performing a regeneration operation to generate electric energy from mechanical energy and capable of storing the electric energy generated by the regeneration operation in the second electrical power storage device; and a control device disposed in the upper turning body to supply a control signal to at least one of the motor generator control unit and the charge/discharge control unit, wherein the control device is arranged to reduce an electrical power storage target value of at least one of the first electrical power storage device and the second electrical power storage device before the regeneration operation is performed by the motor (21, 300, 310), wherein the electrical power storage target value represents a target voltage.