A hydraulic control system of the present invention comprising control valves for controlling the direction and flow rate of pressure oil discharged from a hydraulic pump, hydraulic actuators which the pressure oil is fed to and controlled by the control valves, the hydraulic actuators including hydraulic cylinders and a hydraulic motor, and a first return passage provided as a passage for returning return oil present at the head side of one of the hydraulic cylinders to the tank, the first return passage being in communication with the tank, and a second return passage for returning return oil from each of the other hydraulic actuators including the hydraulic motor except for the hydraulic cylinders to the tank, the second return passage having a back pressure check valve and a replenishing passage, the replenishing passage being configured so as to provide a back pressure developed by the back pressure check valve to the low pressure side of each of the other hydraulic actuators to prevent cavitations.