High dense marine seismic observation is a key to understand earthquake system beneath oceans. We have been developing long-term ocean bottom seismometers (OBSs), which are able to store more than a year continuous seismic records on sea bottoms, since end of the 20th century, and they enable us to acquire many earthquake data when we have a high dense seismic network in marine region. In the last decade, we have done intensive observations in the Japan Trench and the Nankai Trough subduction zones. These experiments reveal seismic activities there precisely, and the results are useful for further studies such as structures of the earth's interior and relations between earthquakes and structural heterogeneity. Otherwise, it remains some problems based on the system. We should use it and other system jointly depending on situations as well as improve our system for future.