To improve engine efficiency in a hybrid construction machine driving an electric power generator by an engine and driving a load by an electric motor supplied with electricity from the electric power generator and an energy storage device.

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 rotation speed switching dial 41 for allowing an operator to arbitrary change an engine rotation speed. 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 a dial value of the rotation speed switching dial 41, 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 13 becomes the electric power generator target output Pgs.