A large plate includes a first main surface and a second main surface, and is separated into a first small plate and a second small plate at a separation surface. The separation surface intersects with the first main surface and the second main surface at a first intersection line and a second intersection line, respectively. The first intersection line and the second intersection line include a curved portion. The first intersection line is disposed on one side of the second intersection line in a planar view. In a cross-section perpendicular to the first intersection line, the separation surface is inclined with respect to a normal to the first main surface. (1) Form a modified portion on the separation surface to be separated. (2) Form a crack on the separation surface. (3) Separate the first and second small plate.