1. A three-dimensional netted structure having an upper surface, a lower surface, two side surfaces, a left end surface, and a right end surface, the structure comprising a plurality of filaments helically and randomly entangled and thermally bonded together, wherein:
said filaments are formed out of a thermoplastic resin by extrusion molding followed by cooling in a liquid;
the structure is four-surface molded, the upper surface, the lower surface and the two side surfaces being molded;
the structure has a pattern of sparse and dense portions arranged in surfaces, wherein four surfaces of an outer peripheral region of the structure that are in parallel to an extruding direction have a higher density than a density of remaining portions of the structure; and
said pattern of sparse and dense portions is formed by cooling in a liquid.