A hybrid construction machine, comprising: a hydraulic actuator; a hydraulic pump configured to discharge hydraulic fluid for driving the hydraulic actuator; a control valve configured to supply the hydraulic fluid discharged from the hydraulic pump to the hydraulic actuator, which is operated by a remote control valve; a generator-motor configured to perform an electric generator action of generating electric power and to perform 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 is configured to cause the generator-motor to perform the electric generator action to charge the electrical storage device and to cause the generator-motor to perform the electric motor action by electric power discharged from the electrical storage device to assist the engine, characterized in that the controller is configured: to cause the generator-motor to perform the electric generator action to charge the electrical storage device in a case of a required pump output, which is an output required of the hydraulic pump, no greater than a predetermined engine lower limit output so as to keep an engine output in a range of no lower than the engine lower limit output; and to set an engine upper limit output higher than the engine lower limit output, and to cause the generator-motor to perform control of the electric motor action to assist the engine in a case of the required pump output no lower than a predetermined engine upper limit output so as to keep the engine output in a range of no higher than the engine upper limit output. a hydraulic actuator; a hydraulic pump configured to discharge hydraulic fluid for driving the hydraulic actuator; a control valve configured to supply the hydraulic fluid discharged from the hydraulic pump to the hydraulic actuator, which is operated by a remote control valve; a generator-motor configured to perform an electric generator action of generating electric power and to perform 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 is configured to cause the generator-motor to perform the electric generator action to charge the electrical storage device and to cause the generator-motor to perform the electric motor action by electric power discharged from the electrical storage device to assist the engine, characterized in that the controller is configured: to cause the generator-motor to perform the electric generator action to charge the electrical storage device in a case of a required pump output, which is an output required of the hydraulic pump, no greater than a predetermined engine lower limit output so as to keep an engine output in a range of no lower than the engine lower limit output; and to set an engine upper limit output higher than the engine lower limit output, and to cause the generator-motor to perform control of the electric motor action to assist the engine in a case of the required pump output no lower than a predetermined engine upper limit output so as to keep the engine output in a range of no higher than the engine upper limit output. to cause the generator-motor to perform the electric generator action to charge the electrical storage device in a case of a required pump output, which is an output required of the hydraulic pump, no greater than a predetermined engine lower limit output so as to keep an engine output in a range of no lower than the engine lower limit output; and to set an engine upper limit output higher than the engine lower limit output, and to cause the generator-motor to perform control of the electric motor action to assist the engine in a case of the required pump output no lower than a predetermined engine upper limit output so as to keep the engine output in a range of no higher than the engine upper limit output.