To improve production efficiency of a glass substrate, by performing chamfering process without positioning the glass substrate.

When the glass substrate G is placed on a table 12, the glass substrate G is sucked and fixed to the table 12 by suction operation of the table 12 without positioning the glass substrate. Next, position information on end surfaces G1, G2 and G3 of the glass substrate G in such a state is acquired by laser displacement gauges 14-22, and a processing start position and a processing finish position of taking into consideration this position information and a desired grinding margin, are calculated by a CPU 28. Next, a motion controller 32 controls moving mechanisms 34 and 40 so that chamfering grinding wheels 24 and 26 linearly move toward the processing finish position from the processing start position based on the position information on the calculated processing start position and processing finish position.