A work W is rotated on a lower grinding wheel 13 provided on a work support base 11. Both surfaces of the work W are simultaneously subjected to grinding process by an upper grinding wheel 24 provided on a grinding shaft 22 of a grinding head 21 and the lower grinding wheel 13. The work W is applied with a pressure force of a primary load through the grinding shaft 22 by a piston rod 36 of a primary pneumatic cylinder 35 which is operated by a first air pressure. The work W is also applied with a pressure force of a secondary load through a lever member 43 by a piston rod 46 of the secondary pneumatic cylinder 45 which is operated by a second air pressure higher than the first air pressure. By adjusting the second air pressure, the pressure force is controlled.