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, said 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;
said structure is four-surface molded, the upper surface, the lower surface and the two side surfaces being molded;
said structure has a first pattern of sparse and dense portions arranged alternately in a direction in which the resin is extruded wherein said structure has a single or multiple high-density regions arranged in a direction of width of said structure;
said structure has a second pattern of sparse and dense portions wherein all surfaces of an outer peripheral region of said structure that are in parallel to the direction in which the thermoplastic resin is extruded have a higher density than a density of remaining portions of said structure;
said first pattern of sparse and dense portions and said second pattern of sparse and dense portions are formed by cooling in a liquid; and
an apparent density of the sparse portion is between 0.01 and 0.09 g/cm3, and an apparent density of the dense portion is between 0.030 and 0.1 g/cm3, the ratio of the apparent density of the dense portion to the sparse portion is between 2.2 and 8.