To improve engine efficiency in a hybrid construction machine using an engine and an energy storage device as a power source.

A control system in a hybrid construction machine includes: a control apparatus 27 for output of control commands to an engine controller 26 and an electric power generator control device 14; and a charge amount sensor 38 measuring a charge amount of the energy storage device. The control apparatus 27 sets an engine target output Pes and an engine target rotation speed ωs achieving the maximum engine efficiency on the basis of the charge amount of the energy storage device, outputs the control command to the engine controller 26 so that the engine rotation speed becomes the engine target rotation speed ωs, calculates an electric power generator target output Pgs so that the engine output becomes the engine target output power Pes, and outputs the control command to the electric power generator control device 14 so that the output of an electric power generator becomes the electric power generator target output Pgs.