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 Mw = 8.1) and the 1946 Nankai (Mw = 8.4) earthquakes. The seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary. The numerical simulation explained the historic rupture patterns and shows the occurrence of a giant earthquake along the entire Nankai trough, a distance of over 600 km long (Mw = 8.7). The growth process revealed from the simulated slip history in and around the strongly coupled patch is: (1) Prior to the giant earthquake, a small slow event (or earthquake) occurs near the segmentation boundary; (2) this accelerates a very slow slip (slower than the plate convergent rate), at the strong patch, which reduces a degree of coupling; and (3) then a rupture easily propagates through the strong patch when the next earthquake is nucleated near the segmentation boundary, consequently growing into a giant earthquake.