We construct fine-scale three-dimensional P and S wave velocity structures beneath the Kanto region, central Japan, by seismic tomography with a spatial correlation of velocities. The Philippine Sea and Pacific plates subduct beneath the Eurasian plate in this area and are imaged with high velocities. Oceanic crust at the uppermost part of these subducting plates is a low-velocity layer. Low-velocity oceanic crust of the Philippine Sea plate subducts to a depth of approximately 80 km. There are also two low-velocity bodies with relatively high VP/VS ratios of 1.80--1.90 in the mantle wedge above the oceanic crust of the Philippine Sea plate. We speculate that the westernmost of these two low-velocity bodies consists of 20% partially serpentinized peridotite, continuous with gabbro in the oceanic crust of the uppermost Philippine Sea plate, while the eastern body is composed of 30% partially serpentinized peridotite. We trace the subducting oceanic crust of the Pacific plate to a depth of ~120 km. The estimated VP/VS ratio of this layer is 1.85--1.90, which indicates a low probability of molten rock; the gabbroic oceanic crust may have been metamorphosed to garnet-granulite.