We propose a generation mechanism of a giant earthquake of magnitude (M) similar to 9 in subduction zones where only M = 7 similar to 8 earthquakes have been identified and the surroundings of the source areas are sliding aseismically. In an M 9 event, both the M = 7 similar to 8 source areas and the surrounding area rupture seismically and the coseismic slip amount is one order larger than that of M = 7 similar to 8 earthquakes. To reproduce such behavior, we assume that an M 9 earthquake occurrence is the fundamental rupture mode in the subduction zone, and the M 9 source area is modeled as a large fracture energy area whose nucleation size is comparable to the size of the source area. The M = 7 similar to 8 asperities are modeled as smaller fracture energy areas whose nucleation size is smaller than the asperity size. Based on these assumptions, we demonstrate a simple numerical simulation of earthquake generation cycles. The results are qualitatively consistent with the characteristics of the 2011 off the Pacific coast of Tohoku Earthquake and a number of phenomena observed prior to this event.