PURPOSE: To prevent the abnormally high pressure in a motor casing when a motor is driven in the condition where the drain line is not connected and the pressure in the motor casing becomes high by providing a relief valve on the drain line in a hydraulic motor having the drain line.

CONSTITUTION: A hydraulic circuit 1 of a pile driver switches the driving of hydraulic motors 2-4 by switch valves 6-8 interposed between the respective hydraulic motors 2-4 and a hydraulic pump 5. These hydraulic motors 2-4 are separably connected to the switch valve 6 and a drain line 9a to a tank 9. Relief valves 2g-4g are provided in the middle of the drain lines 2c-4c of the hydraulic motors 2-4. When the respective hydraulic motors 2-4 are driven without connection to the drain lines 2c-4c and the pressure in a motor casing is increased, the relief valves 2g-4g are opened to reduce the pressure and prevent the abnormally high pressure in the motor casing.