We investigated the spatial distribution of lateral heterogeneity in the upper mantle in the western Pacific region analyzing transverse components of teleseismic P-coda recorded by broad-band seismic networks. Large amplitude transverse components are observed at stations close to plate boundaries such as island arcs for the frequency range of 0.04--2.56 Hz, while small amplitudes are observed at stations on stable continents. Our envelope inversion applied to the observed P-coda reveals large scattering coefficients at depths from 100 to more than 300 km beneath the island arcs, which are interpreted as strong heterogeneities originating from subducting slabs and mantle diapirs. Analyses of a dense seismic network further show a significant difference of heterogeneity between East and West Japan: the former is more heterogeneous than the latter. In contrasts, scattering coefficients less than 20% of the maximum beneath the island arcs are obtained beneath the stable continents, which represents homogeneous and transparent upper mantle.