A hydraulic energy regeneration system for a work machine, comprising: a hydraulic cylinder that contracts during driving of a driven body or an own weight fall of the driven body; a communication pressure boost passage that can boost a pressure of a bottom-side hydraulic chamber of the hydraulic cylinder by communicating the bottom-side hydraulic chamber and a rod-side hydraulic chamber of the hydraulic cylinder with each other during the own weight fall of the driven body; a communication pressure boost valve that is disposed in the communication pressure boost passage and that can regulate one of or both of a pressure and a flow rate of the communication pressure boost passage; a reuse-side line and a reuse control valve that can reuse a hydraulic fluid discharged from the bottom-side hydraulic chamber of the hydraulic cylinder or a regeneration-side line and a regeneration control valve that can regenerate the hydraulic fluid discharged from the bottom-side hydraulic chamber of the hydraulic cylinder as electric energy, during the own weight fall of the driven body; a first pressure sensor that can detect the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder; an operation device that causes the own weight fall of the driven body; an operation amount sensor that detects an operation amount of the operation device; and a controller that is configured to input therein a signal indicating the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder detected by the first pressure sensor and a signal indicating the operation amount of the operation device detected by the operation amount sensor, and that can control the communication pressure boost valve, characterized in that the controller comprises an output regulation section that is configured to add a predetermined delay to the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder detected by the first pressure sensor and reduce an opening degree of the communication pressure boost valve in response to an increase of the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder having the delay right after the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder detected by the first pressure sensor reaches a preset high load set pressure, thereby gradually reducing the opening degree of the communication pressure boost valve with passage of time. a hydraulic cylinder that contracts during driving of a driven body or an own weight fall of the driven body; a communication pressure boost passage that can boost a pressure of a bottom-side hydraulic chamber of the hydraulic cylinder by communicating the bottom-side hydraulic chamber and a rod-side hydraulic chamber of the hydraulic cylinder with each other during the own weight fall of the driven body; a communication pressure boost valve that is disposed in the communication pressure boost passage and that can regulate one of or both of a pressure and a flow rate of the communication pressure boost passage; a reuse-side line and a reuse control valve that can reuse a hydraulic fluid discharged from the bottom-side hydraulic chamber of the hydraulic cylinder or a regeneration-side line and a regeneration control valve that can regenerate the hydraulic fluid discharged from the bottom-side hydraulic chamber of the hydraulic cylinder as electric energy, during the own weight fall of the driven body; a first pressure sensor that can detect the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder; an operation device that causes the own weight fall of the driven body; an operation amount sensor that detects an operation amount of the operation device; and a controller that is configured to input therein a signal indicating the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder detected by the first pressure sensor and a signal indicating the operation amount of the operation device detected by the operation amount sensor, and that can control the communication pressure boost valve, characterized in that the controller comprises an output regulation section that is configured to add a predetermined delay to the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder detected by the first pressure sensor and reduce an opening degree of the communication pressure boost valve in response to an increase of the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder having the delay right after the pressure of the bottom-side hydraulic chamber of the hydraulic cylinder detected by the first pressure sensor reaches a preset high load set pressure, thereby gradually reducing the opening degree of the communication pressure boost valve with passage of time.