This study examined the thermal history of the Kumano forearc basin, as well as slope basin sediments and the accretionary prism, in the Nankai Trough, offshore Japan, using vitrinite reflectance measurements of sediments collected from Integrated Ocean Drilling Program Sites C0004, C0007, and C0009. We detected a paleothermal anomaly in the Kumano forearc basin at Site C0009 that has a 200-m-wide peak with a maximum temperature of 79 °C, ranging from 1000 to 1200 m below seafloor (mbsf). We concluded that thermal fluid is passing through the peak zone based on a curvilinear paleotemperature structure with a wide peak zone. Estimation of reflectance increase through vitrinite reaction promotion suggests that 80–100 °C thermal fluid had passed within at least 100 k.y., thus causing the anomaly. The thermal fluid upwelling could relate to thrusts and ancient splay fault activity near Site C0009. The thermal anomaly zone in the Kumano forearc basin at Site C0009 coincides with the currently active fluid conduit zone imaged on reflection seismic profiles. These results indicate that massive fluid circulation occurs spatially and temporally through thrusts in the subduction zone.