A seismic refraction-reflection experiment using ocean bottom seismometers with artificial sources comprising airguns and explosions was conducted at off Kii Channel, the Nankai Trough, where interplate earthquakes occurred periodically. Using P-wave travel time analyses including non-linear travel time inversion, the depth variation of the crustal structure along the Nankai Trough was revealed. The results show that the subducting depth of the Philippine Sea plate changes at off Kii Channel. The plate is subducting at a shallow location at the west side and at a deep location at the east side. The depth of the plate boundary is about 8 km at the west side and about 9 km at the east side. This result and former studies indicate that the variation of subducting depth exists beneath Kii Channel and off Kii Channel regions. This depth change of the plate boundary might affect the process of interplate earthquake occurrence. ¿ 1998 American Geophysical Union