Experimental monitoring of fluid pressures was initiated in June 2001 at 2 underwaterholes drilled during the Ocean Drilling Program (ODP) Leg 196 to investigate the relationship between deformation and fluid flow processes in the Nankai accretionary prism. ODP Leg 196 visited Sites 1173 (drilled on Leg 190) and 808 (drilled through the frontal thrust on Leg 131), installing 2 Advanced Circulation Obviation Retrofit Kits (ACORKs) to monitor fluid pressures along the walls of the drilled holes. Site 808 penetrates the frontal thrust fault and the déconement, while Site 1173 is located about 11 km seaward from the Nankai Trough deformation front. Formation water freshening around the decollement was first observed on ODP Leg 131, and geochemists have been investigating whether or not the freshening was due to the production of deep-sourced dehydration processes. We now know that the role of smectite dehydration and dehydrant quartz-cristobalite phase transition should be estimated quantitatively. One of the important findings of fluid pressure monitoring is that the formation fluid pressures seem to reflect the change in the stress state in and around the accretionary prism. We believe that such fluid pressure monitoring in the accretionary prism and in the sediments on top of the subducting oceanic plate in terms of stress field and dehydrant process of minerals is a key to deepening our knowledge in future investigations of seismogenic processes.