A hydraulic drive system for construction machines includes a hydraulic pump (1), a plurality of hydraulic actuators (2, 3) driven by a hydraulic fluid supplied from the hydraulic pump, a plurality of flow control valves (4, 5) for controlling flow rates of the hydraulic fluid supplied to the actuators, respectively, and a plurality of distribution compensating valves (6, 7) for controlling differential pressures across the flow control valves, respectively, the plurality of actuators including a first actuator (2) which undergoes a relatively large load pressure and a second actuator (3) which undergoes a smaller load pressure than that of the first actuator. Distribution controllers (22, 23) are provided to control the distribution compensating valve (7) associated with the second actuator (3) such that a differential pressure (Pz2-PL2) across the flow control valve (5) associated with the second actuator (3) becomes larger than a differential pressure (Pz1-PL1) across the flow control valve (4) associated with the first actuator (2), when the first and second actuators (2, 3) are driven simultaneously.