said upper surface, said lower surface, and said two side surfaces are molded; said upper surface, said lower surface, and said two side surfaces are flat; regions of the three-dimensional netted structure, which extend a predetermined distance from said upper surface, said lower surface, and said two side surfaces into an inner portion of the three-dimensional netted structure are compressed; and a density of said regions is higher than a density of said inner portion of the three-dimensional netted structure