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We developed a theoretical method for predicting effective stress and pore pressure based on rock physics model. We applied the method to reveal the pore pressure distribution within the Nankai accretionary prism off southwestern Japan and to investigate variations in pore pressure associated with evolution of the plate boundary décollement. From the crack aspect ratio spectrum estimated from laboratory and well‐log data, we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory. By iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3D tomographic inversion, we estimated in situ mean effective stress within the accretionary prism. Pore pressure is then the difference between the effective stress and the confining stress. When we calculated pore pressure, we considered compressive state of …