Two sheets of nonwoven fabric with the same shape and size each including a rectangular body 11a and a funnel-shaped projection 11b projecting from one side of the body 11a are stacked, and bonded together by heat welding at each side of the body 11a except the projection 11b and both sides of the projection 11b. Joint portions 13a, 13b, 13c, and 13d projecting inward from a joint 13 are located on three sides and four corners of the body 11a except the side provided with the projection 11b, and project to depths at which the joints do not interfere insertion of a nozzle 16 into a center portion of the bag. The nozzle 16 is inserted through an opening 15 of the projection 11b, and the bag is filled with a granular elastic material 12. Then, the projection 11b is removed, and the opening is closed.