The reinforcement of monitoring capability on seafloor is important to understand and forecast the mega-thrust earthquake activity near plate boundary accurately. The deployment of high density real-time seafloor observatory network is suggested to realize this requirement. The maintenance of large scale system extend over a long period of time is one of a great challenge of underwater technology. The conventional high reliability system design of real-time seafloor observatory is not suit for keeping up a large scale system long time. A noble system design concept and practical system management is necessary to make the seafloor observatory network turn into a reality. This paper describes a design concept of the seafloor observatory network that scheduled to install to the Nankai trough in the next several years.