Expected values of the maximum base rock accelerations along coasts of Japan were calculated. 190 points in the coastal area in Japan which include all of the specially designated major and the major ports (129 ports) were selected. The earthquake data in the vicinity of Japan in and after 1885, which are most reliable were used. The maximum base rock acceleration at each point was calculated using Noda and Uwabe's formula which gives the maximum base rock acceleration by the magnitude of an earthquake and the distance. And the expected maximum accelerations at the base rock for the return period were calculated using the extreme value distribution. It is necessary to pay attention to the fact that the expected maximum base rock accelerations discussed here is not equal to the seismic coefficient for the current design of the port and harbour facilities.
　Appendixed show the regional division of the design seismic coefficient for the various design manuals, and the quantitative relationship between importance factor and probability of collapse of structures.