We used a tomographic inversion to determine P and S wave structures in the western Shikoku and Bungo Channel region, for comparison with the locations of low-frequency earthquakes. The velocity model clearly images the high velocity subducting slab and we can see the spatial relation to the low-frequency earthquakes. Under western Shikoku the low-frequency earthquakes occur at depths close to the plate interface, but under the adjacent Bungo Channel region to the west, there is a clear depth separation. Our interpretation is that the low-frequency events are occurring in a region of high Vp/Vs that is located above the subducting slab. The depth separation above the plate seems to vary depending on the region.