Provided is a hybrid construction machine capable of facilitating the hybridization (conversion, redesigning, etc. into a hybrid construction machine) even for construction machines of the minimal tail swing radius type or the minimal swing radius type. A hybrid mini-shovel of the minimal tail swing radius type comprises an engine, a hydraulic pump whose input shaft is coaxially connected to the output shaft of the engine, a generator/motor whose rotating shaft is connected to the output shaft of the engine and the input shaft of the hydraulic pump via a gear mechanism, and a battery which receives and supplies electric power from/to the generator/motor. The generator/motor is placed at a vertical position where its lower end is situated above the shaft center of the input shaft of the hydraulic pump, and when viewed from above at a horizontal position where it overlaps with the hydraulic pump.