Provided is a hybrid construction machine including: a hydraulic actuator; a hydraulic pump which discharges hydraulic fluid for driving the hydraulic actuator; a generator-motor which performs an electric generator action of generating electric power and an electric motor action of generating motive power; an engine connected to the hydraulic pump and the motor-generator; an electrical storage device; and a controller which causes the generator-motor to perform the electric generator action and charge the electrical storage device and causes the generator-motor to perform the electric motor action by electric power discharged from the electrical storage device to assist the engine. The controller causes the generator-motor to start the electric generator action when an engine output becomes equal to or greater than a predetermined engine lower limit output, and controls the electric generator action to keep the engine output no lower than the engine lower limit output. Accordingly, the hybrid construction machine is capable of more reliably preventing the combustion in an engine from deterioration involved by increase in the required pump output.