To provide a hydraulic energy regenerating system for retrieving and using a potential energy of an actuator of an operating machine that is stored by means of hydraulic energy in a returning process of the actuator caused by an operating machine's own weight.

When the operating machine attached to a point of a piston 15 begins to descend by it's own weight, a return oil output from a cylinder chamber 16 passes through a return oil duct 17 to be reduced in pressure by a variable throttle 2, flowing into a flow control valve 3 and then flows into ducts 20, 21 with its flow volume being changed into a predetermined one. A pump motor 4 that has been driven by an engine 9 and has been performing its pumping action begins to be driven in turn by a pressure of the return oil that has flowed into the duct 21, changing its behavior from the pump operation to the motor operation, so that the potential energy stored in the actuator 1 is regenerated.