[1] We conducted permeability measurements on rocks sampled from an exhumed ancient subduction zone fault in the Cretaceous Shimanto accretionary complex of Japan. The permeability under seismogenic environment conditions shows a heterogeneous structure across the fault zone. Permeability of the sandstone in turbidite sequence is the lowest, whereas the highly sheared zone has the largest permeability. A combination of permeability contrast between these two zones and fluid pressure source, including tectonically driven burial and dehydration, could result in a concentration of pore fluids along the fault, which causes Deep Seismic Reflections during underplating stage as delineated in seismic reflection studies. Following a rise of temperature up to 250°C, the permeability of all the sampled rock types became smaller than at room temperature, and also gradually decreased with increasing hold time.