After the great Kanto earthquake of 1923 in Japan, it became necessary to know the earth pressure which acts on the wall during earthquakes, in the reconstruction work of quay walls of Yokohama Harbour. Prof. Mononobe and Dr. Okabe proposed an idea to compute the pressure. They assumed that the resultant force due to max seismic acceleration and acceleration of gravity, acts statically, and adapting Coulomb's theory, computed the seismic earth pressure. According to this computation, intensity of the earth pressure, to be added to static pressure, increases linearly as the depth. But the experiment shows that this pressure is larger near the surface than near the bottom, contrary to the result above mentioned. The authors here propose a new formula for it, and comparing with the results of their experiments, conclude that they bear good resemblance to each other.