Abstract: Buckling plates are provided at comers or sides of a base plate. Each bucking plate selectively buckles between a fixing position, at which a corner or side of a sheet placed on the base plate is fixed, and a release position, at which the corner or side of the sheet is released. This reduces the number of parts and simplifies the structure, thus reducing cost. Further, this simplifies operation and fixes the sheet on a flat surface of the base plate in a stable manner.