A construction machine including a hydraulic drive system that comprises a prime mover, a main pump of the variable displacement type driven by the prime mover, a plurality of actuators (5 to 12) driven by hydraulic fluid delivered from the main pump, a plurality of flow control valves (26a to 26h) that respectively control the flow of the hydraulic fluid supplied from the main pump to the actuators, a pump control device that performs load sensing control for the delivery flow rate of the main pump so that delivery pressure of the main pump becomes higher than maximum load pressure (PLmax) of the actuators by target differential pressure (Pa), a hydraulic fluid supply line (2a, 4a) for supplying the hydraulic fluid from the main pump to the flow control valves, and a tank characterized in that the hydraulic drive system further comprises: a control hydraulic line connecting the hydraulic fluid supply line to the tank; a hydraulic motor arranged in the control hydraulic line so that the hydraulic motor is drivable by the hydraulic fluid delivered from the main pump; a generator connected with the rotating shaft of the hydraulic motor; a pressure sensor that detects the maximum load pressure (PLmax); a control device that performs power generation control of the generator so that the hydraulic motor is driven by the hydraulic fluid delivered from the main pump when the delivery pressure of the main pump becomes higher than a target control pressure (Pun) determined by adding a preset value (Pb) to the maximum load pressure detected by the pressure sensor and an electricity storage device that stores electric power generated by the generator. a hydraulic fluid supply line (2a, 4a) for supplying the hydraulic fluid from the main pump to the flow control valves, and a tank characterized in that the hydraulic drive system further comprises: a control hydraulic line connecting the hydraulic fluid supply line to the tank; a hydraulic motor arranged in the control hydraulic line so that the hydraulic motor is drivable by the hydraulic fluid delivered from the main pump; a generator connected with the rotating shaft of the hydraulic motor; a pressure sensor that detects the maximum load pressure (PLmax); a control device that performs power generation control of the generator so that the hydraulic motor is driven by the hydraulic fluid delivered from the main pump when the delivery pressure of the main pump becomes higher than a target control pressure (Pun) determined by adding a preset value (Pb) to the maximum load pressure detected by the pressure sensor and an electricity storage device that stores electric power generated by the generator. a control hydraulic line connecting the hydraulic fluid supply line to the tank; a hydraulic motor arranged in the control hydraulic line so that the hydraulic motor is drivable by the hydraulic fluid delivered from the main pump; a generator connected with the rotating shaft of the hydraulic motor; a pressure sensor that detects the maximum load pressure (PLmax); a control device that performs power generation control of the generator so that the hydraulic motor is driven by the hydraulic fluid delivered from the main pump when the delivery pressure of the main pump becomes higher than a target control pressure (Pun) determined by adding a preset value (Pb) to the maximum load pressure detected by the pressure sensor and an electricity storage device that stores electric power generated by the generator.