The genesis of the forearc magmatism in southwest Japan at 14 Ma was studied using geologic and seismic observations. Before the magmatism, the Shimanto accretionary complexes were uplifted by 1000–3000 m between 21 and 17 Ma during the opening of the Japan Sea and the Shikoku Basin. Opening of the Japan Sea and the Shikoku Basin terminated at 15 Ma, when the Kinan Seamount Chain on the Shikoku Basin activated. The magmatic products are distributed at segment boundaries and in aseismic areas of the subducting Philippine Sea plate. The segment boundaries are located on syncline and anticline axes of the subducting slab at Kyushu. The magmatic products at Shikoku are distributed at places where olistostromes uplifted between 21 and 17 Ma. Beneath the Kumano volcanic rocks at Kii, a significant discontinuity in the locations of deep earthquakes is observed. These observations indicate that rupturing of the subducting slab by the load of the overriding plate occurred at around 14 Ma. The slab rupturing would have enabled subslab asthenosphere and/or magma to be injected into the plate interface through the tear and cause the forearc magmatism. Since the oceanic plate has a number of preexisting weaknesses, such as fracture zones, slab rupturing could occur more commonly than previously considered. The forearc magmatism caused by slab rupture is an important process associated with the growth of continental crust in subduction zones.