We infer stresses in the subducting slab beneath southwest Japan from focal mechanisms and discuss their geodynamical implications. The majority of the 824 fault plane solutions used in this study were obtained by the Japan Meteorological Agency for earthquakes that occurred during 1970--2002 and were previously unpublished. The focal mechanisms are believed to reflect stresses in both the untransformed slab crust and the slab mantle. The stresses are controlled by tectonic forces, and the earthquakes are facilitated by local dehydration. The slab experiences a regional E--W stretch, that is, downdip and margin-parallel tension at the Kyushu and Nankai margins, respectively. We propose that the margin-parallel component of mantle resistance to the obliquely subducting young slab at Nankai provides an easterly stretch, acting against the westerly slab pull of the older and colder part of the slab at northern Kyushu. Despite the complex slab geometry along the Nankai margin the direction of tension is largely parallel with the local strike of the slab, indicating that the slab is a stress guide. The complex slab geometry is not related to the present stress regime but likely reflects severe deformation in the past when an extremely young and incompetent plate was forced to subduct against its own buoyancy. The maximum margin-parallel stretch and a sharp bending of the slab both occur in the area of the 2001 M 6.7 Geiyo earthquake. Using a simple strength envelope model, we demonstrate that normal-faulting failure of the slab mantle can occur under such conditions.