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A giant earthquake occasionally occurs in a subduction zone owing to a simultaneous rupture in adjacent segments which have been previously ruptured by large earthquakes. However, it is still unknown if a giant earthquake coincidentally occurs, or if there is a causal factor to control its generation. In this study we show a cause and a growth process of a giant earthquake which may occur along southwestern Japan, on the basis of seismic images obtained from wide‐angle seismic data and a numerical simulation incorporating the structural images. The wide‐angle seismic data were acquired along three trough parallel profiles crossing the rupture segmentation boundary between the 1944 Tonankai (moment magnitude M_w = 8.1) and the 1946 Nankai (M_w = 8.4) earthquakes. The seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary. The …