We construct a detailed image of velocity discontinuity beneath southwest Japan by using receiver functions. We first propose an improved receiver function estimation method based on the statistical multivariate autoregressive model. Then we apply the new method to the teleseismic waveform data recorded by the high-density seismograph network in southwest Japan. The results show a clear velocity discontinuity at 30 km depth beneath the southern Shikoku region. This discontinuity, corresponding to the boundary between the oceanic crust and the high-velocity layer (the oceanic Moho) of the Philippine Sea plate (PHS), continues down to the north indicating that the aseismic PHS extends to the central Chugoku region. The continental Moho is also clearly imaged beneath the Chugoku region. The depth contour of the PHS shows a rather complicated feature with ridges and valleys. The most significant ridge is located around longitude 133¿E, west of which the contours are basically directed north-south, changing to west-east to the east. Beneath the western part of this ridge, hypocenters of microearthquakes are located above this velocity discontinuity, while earthquakes mainly occur below the discontinuity, within the oceanic mantle, beneath the eastern part of the ridge.