Oshima-Oshima is a small volcanic island southwest off Hokkaido in Japan Sea. The violent eruption on August 29, 1741 caused a sector collapse of the north flank. Very destructive tsunami with 2000 casualties was also documented, but the generation mechanism has been controversial; volume of subaerial sector collapse, about 0.4 km3, was considered too small to generate such a devastating tsunami. We compile and analyze recently-obtained swath bathymetry data around the island. The volcano is more than 2.3 km high from ocean bottom with its base diameter of 18 km. The total slide volume, both subaerial and submarine, is estimated as 2.4 km3, nearly an order of magnitude larger than the subaerial part. To the north of island, hummocky terrain of debris deposits extends in an area of 69 km2 with the total volume of 2.5 km3, a value similar to the total slide volume. It is one of the largest debris avalanches in historic time, and probably responsible to the tsunami generation. ¿ 2001 American Geophysical Union