A 30 km long offset reflection survey with direction of NE-SW and four expanding spread proflings (ESPs) across the long offset line were carried out at the northwestern part of the Sagami Bay. The experiments were designed to obtain accurate velocity models and to identify the faults related to the forecoming great earthquakes proposed in this region. P-wave velocity models deduced from ESPs show significant differences among profiles and indicate that the crustal structure varies along the long offset line. The obtained velocities were used for the depth conversion of the long offset reflection data. The resultant depth section shows clear reflections dipping from the Izu Peninsula to the Sagami Trough axis. Our records do not indicate the existence of the West Sagami Basin Fracture, one of the most probable earthquake faults, in the rage shallower tan 5 km.