We carried out receiver function imaging to estimate detailed configurations of the continental Moho discontinuity and the Philippine Sea (PHS) slab beneath southwest Japan. We calculated the receiver functions by an extended-time multiple-taper method. A clear discontinuity at about 30 km depth was found beneath northern Chugoku and Kinki, and we interpreted this as the continental Moho. We also found a northward dipping discontinuity at about 20–40 km depth beneath Shikoku, part of Chugoku and the southern Kii peninsula. We present a new interpretation that this clear boundary is the upper part of the PHS slab. Furthermore, a less prominent northward dipping discontinuity is thought to be the aseismic PHS slab at the depth of about 60 km beneath northern Chugoku and Kinki. Most subcrustal earthquakes occur at the plate boundary and/or within the oceanic crust, and their P axes tend to be oriented at high angles to the strike of the PHS slab. In northern Chugoku, existence of the aseismic PHS slab is consistent with the adakitic rocks, which are generated by the melting of younger slabs.