We presented a geodetic observation system, which can provide the seafloor deformation associated with submarine large earthquakes causing tsunamis. In the presentation, we argued that the observed data provided immediate after the earthquakes would improve reliability of real-time tsunami forecasting by explaining the mechanism of tsunami generation. The most important role of earth-scientific observation is to convey information regarding the actual situation of the solid earth and ocean, impacted by massive earthquakes, so that we can anticipate possible types and degree of imminent disasters. We have to stress the importance of strong collaboration with tsunami inundation simulation technology to make the observation data more useful for disaster mitigation. It is also need to be pronounced that the observation technology developed in Japan has to be exported to broad area facing massive earthquakes and tsunamis in the world.