The Ogasawara Plateau, which consists of several seamounts, is situated at the northeastern end of the pacific plate. Its western end reaches the Izu-Ogasawara (Bonin) trench, where many normal faults develop on the plateau. Dives #145 and 146 of ﾒShinkai 6500ﾓ were carried out at the seaward and the landward trench wall near the unction of the Ogasawara Plateau and the trench. At the seaward wall of the trench, we observed the sequence of the reel limestone ranging more than 1,000 m on the lower part of the Ogasawara Plateau. The dive of landward trench wall revealed that the surface is composed of limestone breccias as same as that observed at the seaward side and the stratified tuffaceous sandstone layer which overlies the limestone layer. To interpret the character of the inner trench wall, we can consider two possibilities as follows: 1) The limestones forming the plateau have been fractured as the plateau subducts, and accreted on the inner trench wall, or 29 we observed the east dipping normal fault scarp on the plateau reaching the trench axis. The former interpretation supports the idea that the subduction near the plateau occurs not continuously but eventually, because the sedimentation of the volcanic materials indicates the cease of the subduction-accretion process.