The interseismic strain accumulation process at the Nankai subduction zone, the plate-boundary region between the Philippine Sea and the Eurasian plates off Southwest Japan, has been revealed by GPS measurements spanning five years. Large strains characterized by a contraction of 2.2~3.4¿10-7/yr have accumulated in a narrow region extending parallel to the strike of the plate boundary. The contraction axes lie in a northwest-southeast direction nearly parallel to the direction of relative motion of the Philippine Sea plate with respect to the Eurasian plate. GPS-derived deformation data, including both rate and direction, are consistent with the expected earthquake deformation cycle at the Nankai subduction zone. Current interplate coupling in this zone is strong; much of the relative plate motion has been taken up by deformation of the marginal parts of the Eurasian plate. ¿ American Geophysical Union 1996