In a hybrid construction machine, there is provided a drive control device which realizes in, as a simple a structure as possible, control means 8 of a motor generator 2, which is capable of preventing rapid decreases in the state of charge. The control means 8, which calculates a motor torque command value to be output to the motor generator 2, is characterized by calculating the motor torque command value, based on a difference between an actual state-of-charge and a prescribed target state-of-charge and a torque margin obtained by a difference between a maximum torque of an engine 1 determined based on an actual rotational speed of the engine 1 and an actual torque of the engine 1.