Mononobe-Okabe's equation has been proposed in 1924 and has been used for the aseismic design of the retaining wall, today. But it is well known that the effect of cohesion is not considered in this equation. Author measured a resultant force and a distribution of seismic earth pressure acted on a movable model wall, using a shaking table test. Sand used in the tests is slightly cohesive. It was found that the results agree with the theoretical calculation proposed by the author in 1960. So that the crtical seismic coefficiets for the base sliding of the gravity type quay walls were obtained by using our calculation and it was shown that the seismic damage can be well explained with criterion.