PURPOSE: To enable applying of a continuous pressure and performing of pressure control accurate in good stability, by controlling a pressure of oil by controlling a hydraulic cylinder shaft with fine longitudinal motion.

CONSTITUTION: A pressure valve 9 is opened by a control signal of a CPU7 to start introducing of a pressure of oil. A pressure rise is performed by an oil hydraulic pump 1 driven by the control signal of the CPU7, to perform the pressure rise to a preset pressure of a pressure switch 2, thereafter to end the pressure rise by a control signal output by the pressure switch 2. In stable control of the pressure of oil, in a condition that the pressure of oil is sealed, an output of a pressure sensor 6 is sampled at each fixed time by the CPU7, and after a fluctuation amount of the pressure of oil is calculated, this arithmetic result is converted into a signal of fine moving a hydraulic cylinder shaft 5. A fine moving signal output is transmitted to a motor 4, to longitudinal finely move the hydraulic cylinder shaft 5. By continuously performing this action, the pressure of oil can be stably controlled.