To prevent cullet in chamfering work from sticking to a thin film area by arranging a dustproof cover to seal it by containing the thin film area formed on a glass substrate by bringing the opening edge into close contact with an outer peripheral surface of the glass substrate.

The inside of a dustproof cover 9 is connected to a suction pipe 7 through a connecting tube 10, and is integrally formed with a glass substrate 1 by reducing pressure of the inside in a condition of being installed on the glass substrate 1. A stage 3 moves in this condition, and it is chamfered by performing cutting work on the work edge of the placed glass substrate 1 by rotary grinding wheels 8s and 8b. In this case, since generating cullet does not reach a thin film area 5 of the glass substrate 1 by the dustproof cover 9, the cullet is prevented from sticking to a thin film.