Both a hydraulic motor and an electric motor are provided for driving an upper swing structure. When the amount of electricity stored in a capacitor is appropriate, the upper swing structure is driven by both the hydraulic motor and the electric motor. When the amount of electricity stored in the capacitor exceeds an appropriate value, the mode is changed over to a mode in which the upper swing structure is driven only by the hydraulic motor, and an assist electric generation motor is assist-driven or engine-driven to thereby rapidly return the amount of electricity stored in the capacitor to an appropriate range. The capacity of the capacitor can be reduced, and the amount of electricity stored in the capacitor can be managed without losing the operability of swing motion.