A hydraulic drive system for a construction machine, comprising: a variable displacement first pump device; a second pump device; a plurality of first hydraulic actuators (3a-3g) that are driven by hydraulic fluid delivered from the first pump device; a plurality of second hydraulic actuators (3a, 3e, 3h) that are driven by hydraulic fluid delivered from the second pump device; a plurality of closed center type flow control valves (6b-6d, 6f, 6g, 6i, 6j) that control flows of hydraulic fluid supplied from the first pump device to the first hydraulic actuators(3a-3g); a plurality of open center type flow control valves (6a, 6e, 6h) that control flows of hydraulic fluid supplied from the second pump device to the second hydraulic actuators (3a, 3e, 3h); a plurality of pressure compensating valves (7b-7d), 7f, 7g, 7i, 7j) that control differential pressures across the respective closed center type flow control valves (6b-6d, 6f, 6g, 6i, 6j); and a first pump control unit including a load sensing control section (112a, 112b, 112c) that controls capacity of the first pump device such that a delivery pressure of the first pump device is higher by a target differential pressure than a maximum load pressure of the first hydraulic actuators (3a-3g), wherein the first (3a-3g) and second hydraulic actuators (3a, 3e, 3h) include at least one first specific actuator as a common actuator, the first hydraulic actuators (3a-3g) include a second specific actuator that is used at relatively high frequency in a combined operation with the first specific actuator, the open center type flow control valves (6a, 6e, 6h) include a first flow control valve that controls a flow of a hydraulic fluid supplied from the second pump device to the first specific actuator, and the closed center type flow control valves (6b-6d, 6f, 6g, 6i, 6j) include a second flow control valve that controls a flow of a hydraulic fluid supplied from the first pump device to the first specific actuator, characterized in that: the first and second flow control valves (6a, 6i) are set to have an opening area characteristic such that, when an operating unit for the first specific actuator is operated up to an intermediate zone of an operating range, only the first flow control valve opens to supply a hydraulic fluid from the second pump device to the first specific actuator and, when the operating unit is operated further from the intermediate zone, both the first and second flow control valves (6a, 6i) open to join and supply hydraulic fluid from the first and second pump devices (102, 202) to the first specific actuator. a variable displacement first pump device; a second pump device; a plurality of first hydraulic actuators (3a-3g) that are driven by hydraulic fluid delivered from the first pump device; a plurality of second hydraulic actuators (3a, 3e, 3h) that are driven by hydraulic fluid delivered from the second pump device; a plurality of closed center type flow control valves (6b-6d, 6f, 6g, 6i, 6j) that control flows of hydraulic fluid supplied from the first pump device to the first hydraulic actuators(3a-3g); a plurality of open center type flow control valves (6a, 6e, 6h) that control flows of hydraulic fluid supplied from the second pump device to the second hydraulic actuators (3a, 3e, 3h); a plurality of pressure compensating valves (7b-7d), 7f, 7g, 7i, 7j) that control differential pressures across the respective closed center type flow control valves (6b-6d, 6f, 6g, 6i, 6j); and a first pump control unit including a load sensing control section (112a, 112b, 112c) that controls capacity of the first pump device such that a delivery pressure of the first pump device is higher by a target differential pressure than a maximum load pressure of the first hydraulic actuators (3a-3g), wherein the first (3a-3g) and second hydraulic actuators (3a, 3e, 3h) include at least one first specific actuator as a common actuator, the first hydraulic actuators (3a-3g) include a second specific actuator that is used at relatively high frequency in a combined operation with the first specific actuator, the open center type flow control valves (6a, 6e, 6h) include a first flow control valve that controls a flow of a hydraulic fluid supplied from the second pump device to the first specific actuator, and the closed center type flow control valves (6b-6d, 6f, 6g, 6i, 6j) include a second flow control valve that controls a flow of a hydraulic fluid supplied from the first pump device to the first specific actuator, the first and second flow control valves (6a, 6i) are set to have an opening area characteristic such that, when an operating unit for the first specific actuator is operated up to an intermediate zone of an operating range, only the first flow control valve opens to supply a hydraulic fluid from the second pump device to the first specific actuator and, when the operating unit is operated further from the intermediate zone, both the first and second flow control valves (6a, 6i) open to join and supply hydraulic fluid from the first and second pump devices (102, 202) to the first specific actuator.