PURPOSE: To reduce the volume of the working fluid (the tank capacity), and to prevent the deterioration of the working fluid and the packing by providing an oil cooler in a hydraulic circuit of a locking device, and keeping appropriate temperature of the working fluid.

CONSTITUTION: An oil cooler 7 for cooling the working fluid is provided in the hydraulic circuit to connect an oil chamber 14 of a locking cylinder 3 to an operation cylinder 4 through an oil flow rate adjusting valve 5. This cooler 7 is of multi-tube heat exchanger consisting of stainless steel high pressure tubes, runs oil inside the tubes while it runs the cooling water outside the tubes, and is provided with a plurality of blind plugs for cleaning in upper and lower parts. A stop valve is preferably provided on the inlet side of the cooling water to control the temperature rise of the oil. Because the temperature of the working fluid reaches 80-90°C while a press is operated, and the working fluid is of the pulse intermittent flow of 6-8 shots/min. where the pressure is dropped from 60-70kg/cm2 to 1.3-1.5kg/cm2 instantaneously, the maximum proof pressure of the oil cooler 7 is preferably 230kg/cm2.