Nonwoven sheet, process for producing the same, and filter

A nonwoven sheet contains a substrate layer formed from a fiber aggregate nonwoven structural member; the fiber aggregate nonwoven structural member containing a thermal adhesive fiber; the thermal adhesive fibers are melt-bonded to fix the fibers of the member. The average thickness of the substrate layer is adjusted to not less than 0.2 mm to less than 1 mm, and the thermal adhesive fibers are substantially uniformly melt-bonded in a surface direction of the substrate layer. The sheet may have a surface layer over at least one side of the substrate layer, the surface layer may contain a fiber aggregate nonwoven structural member having an apparent density higher than the apparent density of the substrate layer. The surface layer may comprise a layer formed by heat-pressing or may be formed from a meltblown nonwoven fabric. The thermal adhesive fiber may be substantially uniformly melt-bonded in a thickness direction of the substrate layer. The thermal adhesive fiber may contain an ethylene-vinyl alcohol-series copolymer; the copolymer may form a continuous area of a surface of the fiber. The sheet has an improved flexural rigidity, less deformation under a load, and an excellent formability, in spite of a small thickness thereof.