The breadth and width of a sheet-like filler material before it is placed in a cell of a honeycomb material is set greater than those of the honeycomb material, and after the placement, peripheral edges of the honeycomb material are fringed with filler materials to prevent a filler material from falling out of an end section cell. A liquid adhesive agent is applied to the top of cell walls facing one surface of the honeycomb material, and a surface material is pressed to that surface of the honeycomb material to which the adhesive agent is applied. Then, before the adhesive agent hardens, water-absorbing foam materials are placed in cell spaces by pressing them into the cell spaces from the other surface of the honeycomb material until they are in contact with the adhesive agent. The foam material is quickly adhered to the honeycomb material.