It has been recognized recently that large waves tend to form a group in random sea waves. Overtopping tends to occur particularly when a group of high waves attacks a sea wall. If the capacity of a storage reservoir inside the sea wall is not sufficiently large enough to store a total amount of overtopping brought about by a single group of consecutive high waves, and if a drainage facility is not large enough to pump out sufficient water from the storage reservoir before the next overtopping starts, there is a danger of flooding inside the sea wall. Hence, storage and drainage facilities should be planned to be able to cope with the total amount of overtopping produced by a single group of high waves which overtop the sea wall consecutively. The term "short-term overtopping" referred in this study is that caused by a single group of high waves (see Fig.l). This study aims to clarify the following points: (1) the statistical properties of the amount of short-term overtopping, (2) the method to evaluate a security factor inside a sea wall against flooding by overtopping and an extension of the theory to the short-term overtopping from a comparatively long sea wall.