More than 50,000 high-accuracy arrival times from 1200 shallow and deep earthquakes selected from the Japan University Network Earthquake Catalog are used to investigate the three-dimensional (3-D) seismic velocity structure beneath Japan. Travel times and ray paths are accurately calculated by using an efficient 3-D ray-tracing technique. P wave tomographic images of the crust and upper mantle to a depth of 500 km beneath the Japan Islands are determined with spatial resolutions of 25--50 km. The obtained P wave velocity in the crust and uppermost mantle is slightly higher and relatively homogeneous in western Japan, in contrast with an average lower velocity and a large heterogeneity in eastern Japan where many active volcanoes exist. This feature correlates well with the distributions of heat flow and gravity anomalies. Low-velocity (low-V) zones are imaged in the crust and uppermost mantle beneath active volcanoes. The low-V zones in the mantle wedge dip toward the west and extend continuously to a depth of about 200 km. A high-velocity (high-V) zone corresponding to the subducted Pacific plate is delineated. The thickness of the high-V zone is estimated to be 80--90 km. In southwestern Japan, the descending Philippine Sea plate is also delineated; its thickness is estimated to be 30 km. ¿ American Geophysical Union 1993