The relationships between mountain building, surface erosion, sediment supply to the trench, and growth of the accretionary prism are examined in southwest Japan and the Nankai Trough. Mountain building caused by the subduction of the Philippine Sea plate in the Nankai Trough and collision in central Japan has resulted in a rock uplift rate of ∼4 mm/yr. Surface denudation rates in the mountain regions are on the order of 3–4 mm/yr, resulting from the heavy rainfall of the Asian monsoon. This fact suggests that mountain building is almost in an equilibrium stage in which surface erosion and rock uplift balance...