Seismic responses of a ground with various types of sinusoidal-shaped irregularity were evaluated by using the boundary element method and analyzed based on "scattered-wave contribution". The examination of the numerical results revealed the following: 1) the response at the trough varies sharply with frequency whereas the response at the peak shows large amplification over a wide frequency range; 2) the predominant frequencies at the trough depend on the amplitude of the surface irregularity and do not depend on the number of fluctuations of the ground; and 3) these characteristics can be understood as the results of the interference of scattered waves coming from the irregular surface.