A hydraulic drive system for a construction machine, comprising: a variable displacement type hydraulic pump; a plurality of actuators (5, 6, 7, 8, 9, 10, 11, 12) which are driven by hydraulic fluid delivered from the hydraulic pump; a plurality of flow control valves (26a ∼ 26h) which control flow rates of the hydraulic fluid supplied from the hydraulic pump to the actuators; a plurality of operating devices (34a, 34b, 34c, 34d, 34e, 34f, 34g, 34h) provided corresponding to the actuators and including remote control valves (34a-2, 34b-2, 34c-2, 34d-2, 34e-2, 34f-2, 34g-2, 34h-2) for generating operation pilot pressures for driving the flow control valves; a plurality of pressure compensating valves (27a ∼ 27h) which respectively control differential pressures across the flow control valves; and a pump control system which performs load sensing control on displacement of the hydraulic pump so that delivery pressure of the hydraulic pump becomes higher than maximum load pressure of the actuators by a target differential pressure, the pressure compensating valves being of the type in which full closing of the valves is not attained at the stroke end in the direction of decreasing the opening area, characterized in that the hydraulic drive system comprises a pilot primary pressure circuit (40, 40A, 40B, 40C, 40D) which supplies pilot primary pressure, as pressure of a pilot hydraulic pressure source, to the remote control valves of the operating devices, and the pilot primary pressure circuit includes a first circuit which supplies the pilot primary pressure to the remote control valves (34a-2, 34b-2) of one or more specific operating devices (34a, 34b) among the plurality of operating devices corresponding to one or more specific actuators (5, 6) and a second circuit (42, 42A, 42B, 42C, 42D) which supplies the pilot primary pressure to the remote control valves (34c-2, 34d-2, 34e-2, 34f-2, 34g-2, 34h-2) of operating devices (34c, 34d, 34e, 34f, 34g, 34h) other than the specific operating devices, and when the specific operating devices are not operated, the second circuit supplies the pilot primary pressure directly to the remote control valves of the operating devices other than the specific operating devices, and when the specific operating devices are operated, the second circuit reduces the pilot primary pressure and supplies the reduced pilot primary pressure to the remote control valves of the operating devices other than the specific operating devices. a variable displacement type hydraulic pump; a plurality of actuators (5, 6, 7, 8, 9, 10, 11, 12) which are driven by hydraulic fluid delivered from the hydraulic pump; a plurality of flow control valves (26a ∼ 26h) which control flow rates of the hydraulic fluid supplied from the hydraulic pump to the actuators; a plurality of operating devices (34a, 34b, 34c, 34d, 34e, 34f, 34g, 34h) provided corresponding to the actuators and including remote control valves (34a-2, 34b-2, 34c-2, 34d-2, 34e-2, 34f-2, 34g-2, 34h-2) for generating operation pilot pressures for driving the flow control valves; a plurality of pressure compensating valves (27a ∼ 27h) which respectively control differential pressures across the flow control valves; and a pump control system which performs load sensing control on displacement of the hydraulic pump so that delivery pressure of the hydraulic pump becomes higher than maximum load pressure of the actuators by a target differential pressure, the pressure compensating valves being of the type in which full closing of the valves is not attained at the stroke end in the direction of decreasing the opening area, characterized in that the hydraulic drive system comprises a pilot primary pressure circuit (40, 40A, 40B, 40C, 40D) which supplies pilot primary pressure, as pressure of a pilot hydraulic pressure source, to the remote control valves of the operating devices, and the pilot primary pressure circuit includes a first circuit which supplies the pilot primary pressure to the remote control valves (34a-2, 34b-2) of one or more specific operating devices (34a, 34b) among the plurality of operating devices corresponding to one or more specific actuators (5, 6) and a second circuit (42, 42A, 42B, 42C, 42D) which supplies the pilot primary pressure to the remote control valves (34c-2, 34d-2, 34e-2, 34f-2, 34g-2, 34h-2) of operating devices (34c, 34d, 34e, 34f, 34g, 34h) other than the specific operating devices, and when the specific operating devices are not operated, the second circuit supplies the pilot primary pressure directly to the remote control valves of the operating devices other than the specific operating devices, and when the specific operating devices are operated, the second circuit reduces the pilot primary pressure and supplies the reduced pilot primary pressure to the remote control valves of the operating devices other than the specific operating devices.