PURPOSE: To decrease force for holding a workpiece by a pair of bell clamp holders and to hardly deform or wear the edges of the bell clamp holders by maintaining the amount of displacement of the workpiece at zero during the start of working and during working.

CONSTITUTION: A lens 1 is gripped between a pair of bell clamp holders 2 and 3 and forced to rotate. A rotary grindstone 4 is pressed to the lens 1 to perform center and edge process of the lens 1. At this time, the lens 1 is pressed toward an opposite direction to the pressing direction of the grindstone 4 by pressing rollers 15 and 16. The amount of displacement vertical to the axial directions of the bell clamp holders 2 and 3 is detected by a displacement measuring device. A first and a second motor-driven cylinders 9 and 10 of the pressing rolls 15 and 16 are feedback controlled by a control unit based on the detected values so that the amount of displacement is maintained at zero. Thus, the deformation and abrasion of the edges of the bell clamp holders 2 and 3 are decreased.