Closely spaced seismic reflection profiles obtained from the Nankai accretionary prism provide high resolution images of the prism internal structure. They also provide indirect information about the heat flow estimated from the depths of bottom-simulating reflectors (BSRs) that originate at the gas hydrate phase transition. A careful error evaluation for heat-flow estimation was conducted for this study. More than 40 seismic lines reveal a landward decrease of heat flow closely associated with thickening of the sedimentary section above oceanic basement during prism growth. BSR-estimated heat flow is consistent with heat-flow values obtained by probe as well as expected from the oceanic basement crustal age (15 Ma), except at the prism toe. These data suggest that the thermal structure of the prism is mostly conductive and the effect of fluid flow is relatively small. The oceanic basement surface temperature, extrapolated from thermal gradients and conductivities, is relatively constant (100 to 140°C) for a 30-km-wide zone beneath the seaward part of the prism. Regionally high heat flow occurs at the prism toe, suggesting local advective heat flow transfer. Sidescan sonar images indicate that the advective zone corresponds to regions of rugged topography, where the most active tectonic deformation is presumed to occur by thrust faulting and no slope sediment cover exists. Thus, the locally high heat flow may be caused by localized fluid expulsion related to rapidly increasing tectonic overburden. The existence of mud volcanoes on the trough floor suggests that some pore fluid expelled from the prism migrates seaward through permeable layers.