Strong scattering sources were detected in the upper mantle beneath central Japan from seismic data recorded by the high-sensitivity seismograph network (Hi-net). Anomalous seismograms, consisting of many strong later phases whose amplitudes did not decay significantly with time, were observed from an intermediate-depth earthquake. A comparison of those anomalous seismograms with others generated by other deep earthquakes shows that major scattering sources may exist predominately in the upper mantle, not in the crust. Three possible candidates have been considered as the sources of scattering in the upper mantle including magma conduits, the previous collision zone and slab melting. Although seismic waves may be scattered strongly from each potential candidate, a region associate with slab melting might be more suitable for generating the anomalous seismogram due to the existence of abundant melting spots within the subducted Philippine Sea slab that is extremely young and hot beneath central Japan.