To improve a processing accuracy and processing efficiency of a workpiece by automatically setting hydraulic pressure of a hydrostatic bearing corresponding to the workpiece, a temperature environment and a grinding condition.

After turning power on, material data of a workpiece are transmitted from an operator 80 into a data storage part 60 through a user interface 40. In the same way, temperature data around a grinder are transmitted into the data storage part 60. Then the first grinding condition is roughly set according to contents of a grinding program already stored in the data storage part 60. When the materials data, the temperature data and the grinding data of the work are decided, they are transmitted to a judging part 30, a main shaft stiffness value of a hydrostatic main shaft is determined, and pressure value of a hydrostatic bearing pocket 8 is obtained according to the conditions of three items. Then the pressure value is converted to an electric signal by a grinder control part 70, and is transmitted to a proportional solenoid pressure control valves 3, 4, 5, 6 which control a pressure oil flow by putting back a part of the flow from a pressure oil supply device 2 into a tank 9, and controls a degree of opening of the valves.