Disclosed is a hybrid construction machine including: an engine, a generator-motor, a hydraulic pump, an electric storage device, a power control device which controls electric power between the generator-motor and the electric storage device, a temperature detector which determines a temperature of the electric storage device, an unload valve provided between a discharge port of the hydraulic pump and a tank, an unload-valve control device, and a relief valve. The power control device controls an electric power to be supplied from the electric storage device to the generator-motor, under abnormal circumstances where the temperature of the electric storage device is low, to a power obtained by adding an additional discharge power to a power to be supplied under normal circumstances. The unload-valve control device closes the unload valve, under abnormal circumstances. The relief valve controls the discharge pressure of the hydraulic pump to the set pressure, under abnormal circumstances.