The Nankai trough region, where the Philippine Sea Plate (PHS) subducts beneath the SW Japan arc, is a well-known seismogenic zone of interplate earthquakes. A narrow zone of nonvolcanic tremor has been found in the SW Japan fore-arc [Obara, 2002]. Obara  suggested fluids as a source for tremor. The behavior of fluids at the plate interface is a key factor in understanding fault slip processes. Seismic velocity variations can provide important information on the fluid-related heterogeneous structure. To reveal the subduction structure, we conducted passive seismic experiment in the eastern Kii Peninsula. 90 offline recorders were installed on a 90-km-long line during a six-month period from May, 2015. Arrival times of 275 local earthquakes were used in a joint inversion for earthquake locations and 3D Velocity structure. In order to investigate the relation of the crustal heterogeneity to the seismicity, we relocated 4,720 events occurring between January 1, 2000 and May 26, 2015, including 284 low-frequency earthquakes (LFEs), using the P- and S-wave arrival time data picked manually by the Japan Meteorological Agency and inverted 3D velocity model. We also applied a seismic interferometry technique to the local earthquake data set to obtain the geometry of the subducting PHS. Common-midpoint stacked profile of interferometric seismic imaging shows the northward-dipping reflector at a depth of about 25 km, which is interpreted as the top of the PHS. Most LFEs are located beneath the northward-dipping reflector. LFEs are located in and around the low Vp and high Vp/Vs zone. The low Vp and high Vp/Vs generally suggests the existence of fluid [e.g., Zhao et al., 1996], and indicates the occurrence of the LFEs may be associated with fluids. The epicentral distribution of LFE corresponds to the locked-sliding transition estimated by Hyndman et al. (1995). These studies suggest that fluids dehydrated from the PHS control frictional properties of the plate boundary.