We present a numerical method to analyze the GPS data, and examine slow fault-slips in Hyuga-nada, a region in western Japan where the Philippine Sea plate subducts under the Eurasian plate and earthquakes of Magnitude 7 class are frequent. We found that, after two large earthquakes in 1996, a slow fault-slip expanded from the source area to the north and then triggered another slow event with a characteristic source duration of about one year. The aseismic slip has increasingly highlighted a particular site where little slow fault-slip takes place but where the subducting plate drags the overriding plate. It is noteworthy that the highlighted area is just the site of a past large earthquake. We propose that this area is a possible site for a future large earthquake.