To improve soundproofing performance of a soundproofing wall constituted by wall members overlapping with a hollow portion therebetween.
A resonance state of an outer wall member of a soundproofing wall which includes an inner wall member on the side of a sound generation source, an outer wall member outwards from the inner wall member with respect to a position of the generation source, and a hollow portion which is provided between the inner wall member and the outer wall member is detected. A pressure of the hollow portion is adjusted so that the resonance state is a minimum based on adjustment of the pressure of the hollow portion.