PURPOSE: To attain extremely fine ridgeline forming work by calculating a ridgeline deflection value every rotational angle of a lens and moving the lens on the basis of the ridgeline deflection value.

CONSTITUTION: A frame inner diameter measuring instrument 2b has a probe 28 to be turned while abutting upon a circular inside groove 27 of a frame fixed on the instrument 26 and a rotary encoder 29 for detecting the rotational angle θ of the prove 28 and a distance (inner diameter) R between the rotational center O1 of the prove 28 and the groove 27 is supplied to an arithmetic unit 30 as an electric signal together with the rotational angle θ. The arithmetic unit 30 stores the inner diameters R corresponding to respective rotational angles θ (θ=1W360°) in a measuring table T1 of a memory and calculates the value . The value corresponds to the moving distance of the lens in the axial direction at the time of ridgeline forming work and a pulse motor 23 is controlled by the value .