Hydraulic drive system of construction machine

This hydraulic drive system for construction machinery is provided with: a swing motor; a swing operation device that outputs a swing operation signal according to the inclination angle of an operation lever; a swing direction switching valve that includes a spool and a drive unit which receives a command current and drives the spool, and that increases the supply amount of a working fluid to the swing motor and the discharge amount of the working fluid from the swing motor as the command current increases; a control device that supplies, to the swing direction switching valve, a command current that increases as the swing operation signal increases; and a pressure sensor that detects the outflow pressure of the swing motor. When the swing operation signal has decreased, the control device supplies a command current to the swing direction switching valve when the outflow pressure of the swing motor as detected by the pressure sensor increases past a threshold, thereby maintaining the movement speed of a spool at or below a limit value.