Near Hakodate, the southernmost part of the Oshima peninsula, Hokkaido, a noticeable seismic activity of swarm type has continued since October 23, 1978. Up to the end of 1980, there occurred more than 6, 000 earthquakes larger than M 2, including numerous perceptible ones in and near Hakodate. Three maximum events were of M 4.2 and reached seismic intensity III on JMA scale. The activity is divided into two stages: the first from the beginning to the end of 1978 and the second since 1979. Frequency of earthquake occurrence reached the highest peak, more than one hundred per day, in late October, 1978, in the first stage. From a point of view of energy released by earthquakes, however, the most violent activity appeared in the second stage, especially from middle March to May 11, 1979.In order to locate earthquakes precisely, a temporary seismic network using wireless telemetry was deployed surrounding the focal region. From this observation the following were revealed. In the first stage, earthquakes occurred exclusively off Hakodate (NW region). While in the second stage, earthquakes gave shape to one more focal region (SE region) about 10km southeast from NW region. Most of large events occurred in SE region. Activity in SE region decreased rather monotonously and vanished by April, 1980. On the other hand, activity in NW region has continued in some complicated mode, reviving in August, 1979 and January, 1980.