An object of the invention is to provide a grid assembly which is easy to assemble and is high in assembly reproducibility, and an ion beam etching apparatus including it. A grid assembly is constructed of three grids each in the shape of a circular plate, which are stacked one on top of another. The grid assembly includes three fixing holes for fixing the three grids, and three positioning holes for positioning the three grids. In assembly, the three grids are stacked one on top of another on a first ring so that positioning pins provided on the first ring are inserted into the positioning holes. Then, a second ring is stacked on top of the three grids, and bolts are inserted into the fixing holes. Thus, positioning is performed by using the fixed positioning pins and thereafter the fixing can be performed, which facilitates the assembly.