Provided is a hybrid construction machine, wherein a drive control device is provided which realizes a controller (8) for an electric generator (2) where the controller (8) has as simple a structure as possible and makes it possible to prevent rapid decreases in the amount of charge. The controller (8), which calculates a motor torque command value that is output to the electric generator (2), is characterized in that the motor torque command value is calculated on the basis of: the difference between the actual amount of charge and a prescribed target amount of charge; and a torque margin obtained on the basis of the actual torque of an engine (1) and the maximum toque of the engine (1) determined in accordance with the actual RPM of the engine (1).