The Nankai trough, which runs along the western Japanese islands, is widely known for its systematic activity of large earthquakes with the Magnitudes greater than 7. According to the fault-plane studies of these earthquakes, a major seismic block boundary is supposed to run off the Kii Peninsula almost perpendicular to the trough axis. A seismic experiment was conducted across this boundary region in order to determine seismic crustal structure. P-wave travel time analyses were applied to the data from the OBSs deployed along the trough axis, including travel time nonlinear inversion. The result shows an abrupt change in thickness of the subducting Philippine Sea Plate's crust at the expected location of the seismic block boundary. The crust is thicker in the east by about two kilometers, and the interface between the overriding and subducting plates runs shallower in the east. These structural features may affect the mode of earthquake occurrence. ¿ 1998 American Geophysical Union