Postseismic crustal deformation following the 1993 Hokkaido Nansei-oki earthquake (M = 7.8), northern Japan, was observed by GPS, tide gauge, and leveling measurements in southwestern Hokkaido. As a result of comprehensively analyzing these three kinds of geodetic data, we found the chief cause of the postseismic deformation to be viscoelastic relaxation of the coseismic stress change in the uppermost mantle. Afterslip on the main shock fault and its extension, by contrast, cannot explain the deformation without unrealistic assumptions. The viscoelastic structure, which we estimated from the postseismic deformation, consists of three layers: an elastic first layer with thickness of 40 km, a viscoelastic second layer with thickness of 50 km and viscosity of 4 ¿ 1018 Pa s, and an elastic half-space. This model suggests the presence of an anomalously low-viscosity zone in the uppermost mantle. The depth of the viscoelastic layer roughly agrees with that of the high-temperature portion and the low-Vp zones of the mantle wedge beneath the coast of the Japan Sea in northern Honshu. These correlations suggest that the low-viscosity primarily results from high temperature of the mantle material combined with partial melt and the presence of H2O. These correlations also suggest that the low-viscosity zone in the uppermost mantle is widely distributed in the back arc side of the volcanic front and that postseismic deformation induced by viscoelastic relaxation may be frequently observed after large earthquakes in this area.