We used ambient noise records observed by DONET seafloor seismometer and LTBMS borehole seismometers to monitor seismic velocity structure along the Nankai Trough subduction zone. Seismic interferometry, which can produce zero offset pseudo shot records by calculating auto- and cross-correlation function from ambient noise records, was used. We applied the seismic interferometry method to the ambient noise records observed by DONET and LTBMS seismometers from June to Sep. 2016. We also applied stretch interpolate technique to detect temporal change of seismic velocity during the period. Results suggest that DONET data with seismic interferometry method can detect less than 0.5 % velocity changes below seafloor and LTBMS data can also detect much smaller velocity change which may occur during SSE. These results imply a possibility for this technique to be used as seismic structure monitoring tool without any artificial seismic sources.