PURPOSE: To eliminate impact due to acceleration to a test piece at the time of starting by raising discharge pressure gradually by means of a start-compensation pressure generating device that is provided with a pressure regulating valve and then raising pressure by means of a main pressure generating device when the discharge pressure has reached a set value.

CONSTITUTION: The value of the discharge pressure of a start-compensation hydraulic pressure generating device 12 is made zero, and an auxiliary pump 20 is started. If the discharge pressure is gradually raised by operating the adjusting section 21a of a pressure regulation valve 22 the hydraulic pressure that is adjusted reaches a supply pipe 8 through a check valve 19 and it is supplied to a servo valve 4. When a pressure detector detects a set pressure value, a boost pump 15 and a main pump 14 are successively started. When the discharge pressure reaches a set value, it is raised by means of the regulating section 17a of a pressure regulating valve 17 until the discharge pressure reaches a rated pressure. After that, the auxiliary pump 20 is stopped when the pressure in the supply pipe 8 becomes a working set pressure.