Provided is a hydraulic drive system capable of causing a specific action of a main actuator and an action of an optional actuator simultaneously and properly. The system includes: first and second pumps (11, 12); a first main supply fluid line and an optional supply fluid line leading to the first pump (11); a second main supply fluid line leading to the second pump (12); a main manipulating device (31) for the main actuator (7); an optional manipulating device (32) for the optional actuator (8); a bleed-off flow rate regulating section (42) to change the bleed-off flow rate for the second pump (12); and a bleed-off control section to operate the bleed-off flow rate regulating section (42) to change the discharge flow rate of the second pump (12) in accordance with a control operation applied to the main manipulating device (31) and to make a bleed-off flow rate in a specific combined manipulation state where a specific main control operation and an optional control operation are simultaneously performed be smaller than that in a single main manipulation state.