To provide a device for machining a side part, which can highly accurately machine a side part of a rectangular hard and brittle substrate and is excellent in machining stability.

The device for machining the side part includes a center table to be controlled to turn for supporting the center of a workpiece, a pair of side tables on both sides of the center table for supporting an area other than the center of the workpiece, and a tool for machining both opposite side parts of the workpiece supported by the center table and the pair of side tables. The tool is relatively movable in a direction parallel to a side of the side tables. The pair of side tables have an expanding/narrowing device to be controlled to come closer/farther across the center table synchronously with the turning motion of the center table.