This hydraulic drive system (1) for a construction machine is provided with: control valves (3, 32, 33) that are configured such that the opening area of a center bypass passage (3a) decreases gradually in conjunction with an increase in an operation signal; an operation device (4) that outputs the operation signal; a bleed-off line (6); and a bleed-off valve (7) that has a pilot port (7c), to which secondary pressure of an electromagnetic proportional valve (8) is led, and is configured such that a bypass passage (7a) is opened until the operation signal exceeds a first set value, a bypass passage (8a) is closed when the operation signal exceeds the first set value, an unloading passage (7b) is opened when the operation signal exceeds a second set value, and the opening area of the unloading passage (7b) gradually decreases from when the operation signal exceeds the second set value until said signal reaches a third set value.