Active-source seismic surveys have resolved the fine-scale P-wave velocity (Vp) of the subsurface structure in subduction forearcs. In contrast, the S-wave velocity (Vs) structure is poorly resolved despite its usefulness in understanding rock properties. This study estimates Vp and Vs structures of the Nankai Trough forearc, by applying transdimensional inversion to high-frequency receiver function waveforms. As a result, a thin (~1 km) low-velocity zone (LVZ) is evident at ~6 km depth beneath the sea level, which locates ~3 km seaward from the outer ridge. Based on its high Vp/Vs ratio (~2.5) and comparison to an existing seismic reflection profile, we conclude that this LVZ reflects a high pore pressure zone at the upper portion of the underthrust sediment. We infer that this overpressured underthrust sediment hosts slow earthquake activities, and that accompanied strain release helps impede coseismic rupture propagation further updip.