Array of borehole observatories was established in the seafloor in the Nankai Trough seismogenic zone, in the south of Japan where large earthquake and Tsunami generation is expected in the future. These observatories are connected to seafloor cable observation network for continuous data acquisition to allow monitoring of crustal deformation in real-time and to help understand condition of seismogenic plate interface. In each borehole observatories, a series of sensors such as pore-fluid pressure, volumetric strain, and tiltmeter were installed as well as seismometers to gain enough sensitivity to identify episodic small slow slip events in seismogenic plate boundary. Observation data from these borehole observatories revealed existence of recurring slow slip events in off seismically locked zone of Tonankai earthquakes, which occurs rather frequently but small in magnitude (Mw 5.0- 5.6). Dense array of highly sensitive borehole observatories as well as observation of seafloor pressure to correct the effect of oceanographic loading, are needed to identify these small and slow events in the seafloor.