Hydraulic drive system

A hydraulic drive system comprising:

• a hydraulic pump capable of changing a discharge flow rate of a working fluid;
• a meter-in control valve that controls a meter-in flow rate of the working fluid flowing from the hydraulic pump to a hydraulic actuator;
• a meter-out control valve that is provided separately from the meter-in control valve and controls a meter-out flow rate of the working fluid being drained from the hydraulic actuator into a tank;
• an operation device that outputs an operation command;
• a first pressure sensor that detects a drainage pressure of the hydraulic actuator;
• and
• a control device that sets a target meter-out flow rate according to the operation command from the operation device and controls an opening degree of the meter-out control valve on the basis of the drainage pressure detected by the first pressure sensor and the target meter-out flow rate, wherein
• the meter-out control valve drains the working fluid forced out of the hydraulic actuator into the tank according to a command from the control device, and
• the control device sets the target meter-out flow rate by multiplying a meter-out pressure-receiving area of a pressure-receiving part of the hydraulic actuator by a target speed corresponding to the operation command from the operation device, the pressure-receiving part forcing the working fluid out of the hydraulic actuator.