An aim of our research is to develop a new tsunami detection system by using an ocean radar. In this study, a back scattering strength is proposed as the new detection index. The index is examined by using the ocean radars data observed in Ishigaki Island and Yonaguni Island. There were 46 earthquakes occurred from April 1, 2004 to March 31, 2006. For the M6.5 earthquake, the back scattering strength became larger rapidly around its epicenter when the main shock occurred. However, the M6.5 is not large for tsunamigenic earthquakes. Then, larger sea surface disturbance due to Typhoon Matsa in 2005 are also analyzed. Accordingly, it is confirmed that the back scattering strength became larger when it passed over the ocean radar observation region.