There are significant variations in the size, especially lateral extent, of the co-seismic rupture zones and inter-plate locked zones for an inter-plate mega-thrust earthquake. Obvious factors controlling those variations have been still unsolved questions. We designed integrated seismic profiles, including a wide-angle seismic profile and a multichannel reflection seismic profile, at the eastern Nankai trough in central Japan. Refraction tomography and pre-stack depth migration of wide-angle seismic data successfully image a trough-parallel cyclic ridge subduction, with thickness of 13--20 km and wavelength of 35--50 km, at a geodetically determined inter-plate locked zone in a non-rupture zone during the 1944 Tonankai mega-thrust earthquake. This result suggests a first direct evidence for the subducted seamount = strongly locked hypothesis.