PURPOSE: To reduce the contact area between foamed materials which deteriorates its air-permeability when the foamed materials are elastically deformed, by forming, on one of the foamed materials, several projections which are abutted against the other foamed material to form a space.

CONSTITUTION: Outer and inner foamed materials 17a, 17b are superposed one upon onother to form an element 17, and the top ends of several projections 22 formed on the other foamed material 17b, are abutted against the inner foamed material 17a to form a space 23. The element 17 is held by a holding member 18 such that the bottom of the element 17 is held between the bottom section of a frame member 16 and a securing member 27, and the inner flange of the element 17 is held between a body section 24 and the opening edge of the frame member 16.