In order to better understand the deformation structures within the thick accreted sediments beneath forearc basin, we apply an advanced beam migration to a legacy 2D seismic data acquired in the deep-water area in the Nankai Trough, Japan. In this region, many seismic surveys have been conducted to study the geophysical and geological features in the seismogenic zone related to the plate subduction. However, the details in the accreted sediments beneath the Kumano forearc basin are still unclear due to the poor seismic images caused by difficult geological structures. The common reflection angle migration is an advanced depth migration technology and an effective method for imaging such complex structures. By applying this method, deeper reflections that were difficult to image previously are clearly revealed. The newly imaged geometric features (e.g., the folds in the shallow part and the deep reflectors with stepwise discontinuities and the land-ward thickening of the lower layer) implies that the deformation is characterized by development of multiple thrust faults in the thick accreted sediments. The common reflection angle migration provides new seismic evidences that are essential for understanding the Nankai Trough seismogenic zone and the accretionary prism development.