Provided is a method of controlling the regenerating condition by use of a construction machine having an upper revolving structure with a hybrid drive unit, and a lower traveling body, wherein when in a brake mode, the hydraulic motor is neutralized by the internal communication of an inlet port and an outlet port of the hydraulic motor via a communication valve, such that most of the kinetic (inertia) energy is efficiently transduced into electric energy by way of regenerative power generation for electric storage, and such that the switchover between driving and regenerative power generation is rendered in the optimal condition. The control unit determines the control mode of the control unit in accordance with the rotation direction of the hydraulic actuator, the operating direction of the hydraulic actuator controller, the actual rotational velocity of the hydraulic actuator, and the manipulated variable of the hydraulic actuator controller.