PROBLEM TO BE SOLVED: To provide a hydraulic drive system adapted to perform load sensing control, configured in such a manner that, when actuator operation is not performed, particulate matter accumulated on the filter of an exhaust gas purification device is efficiently burnt and removed by pump output increasing control, that, even if the actuator operation and the pump output increasing control are performed at the same time, they do not affect each other, and that the hydraulic drive system is simple and low cost.SOLUTION: The hydraulic drive system includes: a solenoid switching valve 46 switching between tank pressure and the discharge pressure of a pilot pump 30 upstream of an engine rotation speed detection valve 13 and outputting the selected pressure to a shuttle valve 45; and a solenoid switching valve 48 disposed in an oil path 12b for leading the output pressure of a differential pressure reducing valve 11 to an LS control valve 17b and switching between the validity and invalidity of the load sensing control. When the exhaust gas purification device 42 needs regeneration, a controller 49 changes over the solenoid switching valve 46 and the solenoid switching valve 48 so that the former outputs the discharge pressure of the pilot pump 30 as pseudo-negative pressure and the latter invalidates the load sensing control.