The recent development of Global Navigation Satellite Systems (GNSS) and communication infrastructure provides real-time displacement data, enabling the real-time estimation of coseismic fault models for large earthquakes and the potential improvement of tsunami warning systems. In this paper, we present two real-time coseismic fault model estimation procedures implemented in the GEONET real-time processing system: a single rectangular fault model and a slip distribution model on the subducting plate interface. We evaluate the fault modeling procedures for two past large interplate earthquakes: the 2003 Tokachioki earthquake and the 2011 Tohoku earthquake. Furthermore, we also evaluate a potential large interplate earthquake along the Nankai trough based on simulated GPS time series. We obtained reasonable coseismic simple rectangular fault models for the past two large earthquakes. In contrast, the simulated interplate event along the Nankai trough clearly indicates the great advantage of estimation of the coseismic slip distribution on the plate interface. These results clearly show that reliable estimation of the coseismic fault model and/or slip distribution on the subducting plate interface will be possible in Japan based on the real-time GNSS data. This result will contribute to a more reliable tsunami early warning system.