To facilitate corner treatment such opening-closing operation of upper and lower blades to round a corner portion of a brittle plate-shaped material such as plate glass by respectively providing upper and lower blade edge portions of a substantially scissors-shaped cutter body with upper and lower blade bodies in such a manner that bent upper and lower blade edge lines thereof face each other.

A predetermined finish line B1 is marked on a corner 2 of a glass plate 1, and a predetermined cutting line C1 provided with a grinding allowance is set outside of the finish line B1. Then, bent upper and lower blade edge lines 14, 15 of upper and lower blade bodies 9, 10 mounted to upper and lower blade edge portions 5, 6 of a cutter body 8 are matched on the predetermined cutting line C1. Further, a gripping pressure in a direction perpendicular to a surface of the glass plate 1 is generated between the upper and the lower blade edge lines 14, 15 by gripping left and right holding portions 4, 3, whereby cracks are generated at both ends of the predetermined cutting line along the peripheral edge of the glass plate 1 since the gripping pressure is the greatest at these ends, then the cracks pass through the predetermined cutting line to form the corner 2. Thereafter, the corner 2 is finished by grinding with grindstones or the like until the predetermined finish line B1.