PURPOSE: To polish a workpiece in desired thickness by measuring the polished amount of the workpiece by a measuring instrument to thus accurately and exactly measure the polished amount of the workpiece.

CONSTITUTION: When the elevational motion of a measuring element 23 is interlocked with the motion of a polishing point P of a workpiece and the point P reaches its highest point (on the axis of the rotary shaft), the upper end of the element 23 is moved to predetermined position. When the thickness of a lens 17 is reduced thereafter by the polishing work, the point P is moved downwardly, while the element 23 is upwardly moved. When the elevational movement of the element 23 is detected by the measuring instrument 25, the polished amount of the lens 17 is detected at its special point by the polishing tool 16. When the polished amount of the lens 17 reaches predetermined value, it is measured and detected by the instrument 25, and the polishing work is stopped by the tool 16 to obtain the lens 17 having desired radius of curvature.