We determine the three-dimensional P wave velocity structure beneath the Kanto-Tokai district, central Japan, and simultaneously relocated by inverting travel time data for 91 events that were observed 61 stations operated by the National Research Center for Disaster Prevention. The variance of the travel time data is reduced by 75%, as compared to the variance for the routinely used laterally homogeneous model. In general, our velocity model is consistent with tectonic models of the Kanto-Tokai district. Our velocity model is also in good agreement with seismic explosion data and Bouguer gravity anomalies. The uppermost layer (from 0 to 16 km depth) is characterized by lateral velocity differences of more than 20%. We find low-velocity zones beneath Tokyo Bay, the middle of the Boso Peninsula, and Omae-zaki, where there are thick sedimentary layers. A marked 30-km-thick high-velocity zone extends to the northeast and the west of the Izu Peninsula. This zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the Philippine Sea (PHS) plate, which is being subducted at the Sagami and the Suruga troughs. The data unquestionably require a high-velocity zone beneath Mount Tsukuba that cannot be explained in terms of present tectonic models. We use our three-dimensional velocity model to relocate about 23,000 earthquakes that took place from 1983 through 1985. The accuracy of the hypocentral parameters improved considerably. As a result, the double seismic zone within the subducting Pacific plate is more clearly delineated than in previous studies. The inclined seismic zone for the subducting PHS plate coincides fairly well with the high-velocity zone of our three dimensional velocity model. ¿ American Geophysical Union 1988