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 each other, resulting in a constant altitude of ∼2000 m. Several drainage systems on land and in offshore submarine canyons enable the transport of eroded sediments directly into the Nankai Trough. Most of the terrigenous sediments supplied to the Nankai Trough are accreted in the subduction zone of the Philippine Sea plate. The accretion rate of the sediments in the eastern Nankai Trough is ∼1.68 × 10 7 m 3 /yr, which is consistent with the denudation rate of the Akaishi Mountains, contributing to the supply of 1.72 × 10 7 m 3 /yr of sediment in central Japan. The growth of the accretionary prism is an important controlling factor for the onset of large earthquakes in the Nankai Trough, because the hanging wall of the rupture area of the seismogenic zone is composed entirely of the accretionary prism. Repeated large earthquakes with a recurrence time of ∼100–200 yr, which are well recorded in the Nankai Trough, in turn promote surface erosion through consecutive landslides and tsunamis. Southwest Japan, with its extensive record of both erosional processes and seismic events, shows the intimate long-term relationships between tectonically driven mountain building, surface erosion under the Asian monsoon climate, growth of the accretionary prism in the trench, and the generation of large earthquakes.