A hybrid construction machine drives the upper swing structure using a hydraulic motor and an electric motor together and is capable of regenerating the energy of the upper swing structure in deceleration or stopping into electric power and using the regenerated electric power for assisting the hydraulic motor for driving the upper swing structure. Springs coupled to the mass compress in response to the linear movement along either of two opposing directions. The power capacitor also includes at least one mechanical spring element between the housing and the at least one capacitor winding.