PURPOSE: To obtain high sound elimination effect in a wide frequency range by joining the reverse surfaces of honeycomb cores of a cell structure having plural vertically linked cavity parts on a flat plate, charging a filler in the cavity parts stepwise, thus forming Helmholtz's resonance parts which differ in cavity height and sticking surface materials which have holes on the top surfaces.

CONSTITUTION: The reverse surfaces of the honeycomb cores 2 with many vertically linked cavity parts 3 are joined on the flat plate 1 which has constant thickness and a flat surface such as an FRP plate. Then, the filler 4 made of a foamed material is charged in those cavity parts 3 stepwise in order to form the resonance parts 3a - 3e which has desired cavity heights and provide Helmholtz's resonance at mutually difference center frequencies. Then, the surface material 5 which is provided holes 5a is joined on their top surfaces. Thus, the resonance parts 3a - 3e are made to resonate over a wide frequency range, thereby reducing noises.