A hybrid construction machine, comprising: an engine; a motor generator driven by the engine and capable of generating electric power; a hydraulic pump driven by total torque of the engine and the motor generator; a plurality of hydraulic actuators (3a, 3b, 3c) driven by hydraulic fluid delivered from the hydraulic pump; a power storage device that stores electric power generated by the motor generator and supplies electric power for driving the motor generator; an inverter that performs transfer of electric power between the power storage device and the motor generator; a filter that collects a particulate material in exhaust gas of the engine; and a controller that outputs a command signal to the inverter in order to control powering operation or regeneration operation of the motor generator, wherein the controller includes a filter regeneration decision section that decides a demand for regeneration of the filter, a hydraulic load calculation section that calculates a load of the hydraulic pump, a torque command value calculation section that calculates a torque command value for the motor generator, and a powering operation limitation section that outputs a signal obtained by limiting the torque command value in accordance with the torque command value for the motor generator calculated by the torque command value calculation section and a demand signal for regeneration of the filter decided by the filter regeneration decision section, characterized in that the controller includes a powerable amount calculation section that calculates, when the hybrid construction machine is not idling, a powerable amount of the motor generator that is a difference obtained by subtracting a sum of an idle load of the engine and a torque margin value from the load of the hydraulic pump calculated by the hydraulic load calculation section, and the powering operation limitation section is configured to limit the powering operation of the motor generator when the filter regeneration decision section judges that the demand signal for regeneration of the filter is available and the torque command calculated by the torque command value calculation section is greater than the difference obtained by subtracting a sum of an idle load of the engine and a torque margin value from the load of the hydraulic pump, the difference being calculated by the powerable amount calculation section. an engine; a motor generator driven by the engine and capable of generating electric power; a hydraulic pump driven by total torque of the engine and the motor generator; a plurality of hydraulic actuators (3a, 3b, 3c) driven by hydraulic fluid delivered from the hydraulic pump; a power storage device that stores electric power generated by the motor generator and supplies electric power for driving the motor generator; an inverter that performs transfer of electric power between the power storage device and the motor generator; a filter that collects a particulate material in exhaust gas of the engine; and a controller that outputs a command signal to the inverter in order to control powering operation or regeneration operation of the motor generator, wherein the controller includes a filter regeneration decision section that decides a demand for regeneration of the filter, a hydraulic load calculation section that calculates a load of the hydraulic pump, a torque command value calculation section that calculates a torque command value for the motor generator, and a powering operation limitation section that outputs a signal obtained by limiting the torque command value in accordance with the torque command value for the motor generator calculated by the torque command value calculation section and a demand signal for regeneration of the filter decided by the filter regeneration decision section, characterized in that the controller includes a powerable amount calculation section that calculates, when the hybrid construction machine is not idling, a powerable amount of the motor generator that is a difference obtained by subtracting a sum of an idle load of the engine and a torque margin value from the load of the hydraulic pump calculated by the hydraulic load calculation section, and the powering operation limitation section is configured to limit the powering operation of the motor generator when the filter regeneration decision section judges that the demand signal for regeneration of the filter is available and the torque command calculated by the torque command value calculation section is greater than the difference obtained by subtracting a sum of an idle load of the engine and a torque margin value from the load of the hydraulic pump, the difference being calculated by the powerable amount calculation section.