Various damages like break of seawalls, inundation on patrol roads and floods in buildings in the Japanese offshore airports are often induced by the storm surge and wave overtopping due to typhoons these days. One of countermeasures is the raise　of seawall height but high seawall may cause some disturbances to the taking-off and landing in airports.
　Minami et al.(2007) proposed a new type seawall with double parapets and permeable layer to prevent the wave overtopping in the offshore airports. In order to design the new type airport,appropriate determination of the width of permeable layer and height of backward parapet is necessary. In this paper, a series of experiments is carried out to investigate the wave overtoppingrate for various widths and heights of the new type seawall to obtain the suitable dimension. Major results are as follows.
1) Wave overtopping rate becomes remarkably small as the width of permeable layer becomes larger than 6m.
2) Wave overtopping beyond the backward parapet is prevented when
the parapet height become more than 2.5m.
3) The new type seawall is effective to reduce the wave overflow due to long period waves.However the permeable layer much wider than 18m is demanded to protect the inundation in airports.