A control system for a hybrid construction machine includes a turning-regeneration-use switching valve that opens when a pressure detected by a pressure detector reaches a turning-regeneration starting pressure, so as to introduce the working fluid from a turning circuit to a regenerative motor, an operating-state detector that detects an operating state of a fluid pressure cylinder; and a cylinder-regeneration-use switching valve that opens based on a detection result of the operating-state detector, so as to introduce the working fluid from the fluid pressure cylinder to the regenerative motor. When the turning regeneration is performed alone, the working fluid from the turning circuit is introduced into the regenerative motor without pressure reduction, When the turning regeneration and the cylinder regeneration are simultaneously performed, the working fluid from the turning circuit is reduced in pressure, joins the working fluid from the fluid pressure cylinder, and is introduced into the regenerative motor.

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