cross laying machine or the like is cut into approximately square pieces having a dimension of an approximately 1 000 mm x approximately 1 000 mm and these pieces are used as the multilayered fibrous material blocks 50 it should be noted here that it is not always necessary that the multilayered fibrous material 50 is directional the fibers in the multilayered fibrous material 50 may extend in random directions the bulk density of the multilayered fibrous material 50 is desired to be approximately uniform as for the material for the fibers 51 in the multilayered fibrous material 50 a type of fiber structured of two portions core portion 51 a of polypropylene and sheath portion 51 b of polyethylene which sheaths the core portion 51 a as shown in fig2 is used as the fiber in this embodiment hereinafter sheathed fiber the melting point of polypropylene is approximately 180 u00b0 c and the melting point of polyethylene is approximately 130 u00b0 c therefore the difference in melting point between the two materials is approximate 50 u00b0 c as for fiber diameter fiber with a diameter of 5 u03bcm to 50 u03bcm is generally used in this embodiment fiber with a diameter of approximately 18 u03bcm 2 deniers is used although the above described sheathed fiber 51 is used in this embodiment the fiber structure does not need to be limited to this structure for example a blend between pure polypropylene fiber and pure polyethylene fiber or a blend between sheathed fiber formed of polypropylene and polyethylene and pure fiber may be used when the sheathed fiber 51 is used the polyethylene is present at all intersections among.