PURPOSE: To enable a bottom side chamfering size to be worked to a fixed value even when a center thickness size provides unevenness, by determining a bottom side chamfering wheel position of a lens by the reference of a center thickness and a position detected by clamping the lens to a chamfering machine.

CONSTITUTION: A chamfering size arithmetic circuit, fetching a detected signal of a size of center thickness and a position of a lens with a size of width L of a wheel, calculates the position of a bottom side chamfering size Δd2, and if a change quantity signal for a position of the size Δd2 is indicated to a pulse motor 48, the motor 48 rotates to vertically move a screw stopper 44 by a change quantity, determining a position of the stopper 44 for a detent 42, that is, the displacement of a wheel to a bottom side chamfering position. And if a slider 24 is lifted, the detent 42 is adapted to the bottom side chamfering stopper 44, positioning the wheel 28 to the bottom side chamfering position. Next, the bottom side of the concave lens 1 is chamfered by cutting of the wheel 28. Accordingly, the profitable method in an optical design in accordance with the latest technical requirement for a short, thin, light and small lens can be ensured.