The generating mechanism and process of slow earthquakes can help us to better understand the seismogenic process and the petrological evolution of the subduction system, but they are still not very clear. In this work we present robust P and S wave tomography and Poisson's ratio images of the subducting Philippine Sea Plate beneath the Kii peninsula in Southwest Japan. Our results clearly reveal the spatial extent and variation of a low-velocity and high Poisson's ratio layer which is interpreted as the remnant of the subducted oceanic crust. The low-velocity layer disappears at depths >50 km, which is attributed to crustal eclogitization and consumption of fluids. The crustal eclogitization and destruction of the impermeable seal play a key role in the generation of slow earthquakes. The Moho depth of the overlying plate is an important factor affecting the depth range of slow earthquakes in warm subduction zones due to the transition of interface permeability from low to high there. The possible mechanism of the deep slow earthquakes is the dehydrated oceanic crustal rupture and shear slip at the transition zone in response to the crustal eclogitization and the temporal stress/strain field. A potential cause of the slow event gap existing beneath easternmost Shikoku and the Kii channel is the premature rupture of the subducted oceanic crust due to the large tensional force. Plain Language Summary Various phenomena of slow earthquakes have been observed seismologically and geodetically in recent years around the circum-Pacific subduction zones. It has been generally considered that slow earthquakes could be used as a proxy to monitor aseismic slip and precursors to regular earthquakes, and they may play a role in stress transfer from slow to huge earthquakes. However, the generating mechanism and process of slow earthquakes are still not very clear. This study focuses on the process of the subducted oceanic crustal eclogitization and its relationship with the generating mechanism of the slow earthquakes. We suggest that the generating mechanism of the slow earthquakes is the dehydrated oceanic crustal rupture at the transition zone in response to the prograde eclogitization and the stress/strain field. The present results shed new light on the generating mechanism and process of slow earthquakes.