When the main cylinder of the hydraulic machine descends slowly to contact the workpiece, 1Y A a is energized, the upper position of the valve 1 is connected to the system, and the pressure of the hydraulic system is increased. The pressure in the upper chamber of the master cylinder is continuously increased, and the output flow rate of the constant power variable plunger pump is continuously reduced, the pressure is continuously increased, and the workpiece is pressurized. When the stop on the slider covers the travel switch, the pressurization is completed. The electric contact pressure gauge sends out a signal to disable all power, the hydraulic valve is in the neutral position, and the control chamber of the cartridge valve is connected to the fuel tank. Therefore, the cartridge valve is opened and closed, and the upper and lower chambers of the master cylinder are locked to hold the workpiece. The oil output from the hydraulic pump is discharged through the oil return tank and the hydraulic pump of the four-column hydraulic press.