A new tsunami observation system has been developed, which employs the RTK-GPS technique to detect a tsunami before it reaches the coast. After a series of preliminary experimental studies, the operation-oriented experiments were conducted at two offshore sites. These systems succeeded to detect four 10cm tsunamis on 23rd June 2001 Peru earthquake, 26th September 2003 Tokachi earthquake, 5th September 2004 Kii earthquake and 28th February 2010 Chile earthquake. The newly established Muroto GPS buoy system is continuously operating now. The developed GPS buoy system has been adopted as a part of the NOWPHAS by the Ministry of Land, Infrastructure, Transport and Tourism. These GPS buoys successfully recorded the tsunami of the 28th February 2010 Chile earthquake (Mw8.6). These results well substantiate of a GPS buoy to be a powerful tool for early detection of tsunami. On going and future potential development of the system include (1) continuous observing system of ocean bottom crustal deformation using GPS-Acoustic system, (2) observation of tropospheric zenith delay for application to atmospheric research through estimating water vapor content, but its potential capability may not be limited only to these.