A hybrid construction machine, comprising: an engine; a generator-motor which has a function as an electric motor generating motive power for assisting the engine and a function as a generator generating electric power; a hydraulic pump which is driven by the engine and the generator-motor; an electric storage device which is connected to the generator-motor so as to be enabled to store electric power generated by the generator-motor and supply electric power to the generator-motor; an electric-power control device which controls electric power given and received between the generator-motor and the electric storage device; a temperature detector which determines a temperature of the electric storage device, characterized by an unload valve provided between a discharge port of the hydraulic pump and a tank; an unload-valve control device which controls opening and closing of the unload valve; and a relief valve provided between the discharge port of the hydraulic pump and the tank and having a predetermined set pressure, wherein the power control device controls an electric power to be supplied from the electric storage device to the generator-motor, under abnormal circumstances where a temperature of the electric storage device is lower than a predetermined set temperature, 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; and the relief valve controls a discharge pressure of the pump so as to make the discharge pressure be the set pressure, under abnormal circumstances. an engine; a generator-motor which has a function as an electric motor generating motive power for assisting the engine and a function as a generator generating electric power; a hydraulic pump which is driven by the engine and the generator-motor; an electric storage device which is connected to the generator-motor so as to be enabled to store electric power generated by the generator-motor and supply electric power to the generator-motor; an electric-power control device which controls electric power given and received between the generator-motor and the electric storage device; a temperature detector which determines a temperature of the electric storage device, characterized by an unload valve provided between a discharge port of the hydraulic pump and a tank; an unload-valve control device which controls opening and closing of the unload valve; and a relief valve provided between the discharge port of the hydraulic pump and the tank and having a predetermined set pressure, wherein the power control device controls an electric power to be supplied from the electric storage device to the generator-motor, under abnormal circumstances where a temperature of the electric storage device is lower than a predetermined set temperature, 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; and the relief valve controls a discharge pressure of the pump so as to make the discharge pressure be the set pressure, under abnormal circumstances.