The seismogenic zone at subduction zones does not normally extend to the trench axis. The shallowest part of the plate interface is considered to be seismically decoupled. The location of the seaward updip limit of the seismogenic zone and its relation to the crustal structure are important for understanding the transition process from aseismic slip to the seismic rupture at the plate interface. The Nankai Trough seismogenic zone, southwestern Japan, is one of the most well-studied subduction seismogenic zones in the world. However, the offshore seismicity around the Nankai Trough is very low, and hypocenters are not determined accurately by the on-land seismic network. We performed microseismicity observations using ocean bottom seismographs off Cape Muroto. Hypocenters were determined by using a three-dimensional Vp and Vs structure based on seismic survey results. The observed microseismicity seems to be classifiable into two types. The first is earthquakes that occur around the plate interface; the second is a group of earthquakes that occur in the uppermost mantle of the subducting oceanic plate. The seismicity around the plate interface forms several clusters. The seaward limit of the clusters is characterized by earthquakes with a similar waveform. These similar waveform earthquakes are considered to occur at small asperities in the aseismic-seismogenic transition zone of the plate interface. The seismicity in the uppermost mantle may be related to dehydration embrittlement of subducting serpentinized mantle.