In 1978 and 1980, 12 lines of multi‐channel seismic reflection survey were carried out by the Hydrographic Department of Japan in the Nankai, Suruga, and Sagami Troughs bordering the northern subducting margin of the Philippine Sea plate. Multi‐channel seismic profiles on the Nankai Trough and the southern part of the Suruga Trough revealed the typical structure of subduction zone. The subducting oceanic crust is pursued for at least 30 km landward from the axis of Nankai Trough with the inclination of 3°‐5°. In the southener part of the Suruga Trough, the basement forming the Izu Peninsula is traced beneath the Trough and western slope for about 8 km with an inclination of 7°‐23°. On the lower landward slope of the trough distinctly imbricated sediment bodies with many thrust faults and folds, or accretional prism, were found. Probable accretional prism is formed by deformation of the trough‐fill sediments and makes the ridge and trough zone in the lower landward slope. On the other hand, the structure of the basement could not be identified in the northern part of Suruga Trough. However, some characteristics of the profiles suggest the subduction. In Sagami Trough the basement forming the Izu Peninsula is recognized beneath the Trough to the Misaki Knoll located on the landward slope of the Trough, with inclination of about 10°.