A hybrid-type construction machine includes: an engine to drive a variable-volume-type hydraulic pump; a plurality of hydraulic actuators driven by a hydraulic pressure from the variable-volume-type hydraulic pump; and a motor generator driven by an electric power from an electric power accumulator to assist the engine. A maximum output of the variable-volume-type hydraulic pump is equal to or larger than a maximum output of the engine. When operating by setting an output of the variable-volume-type hydraulic pump larger than the maximum output of the engine, the engine is assisted by the motor generator.