Analysis of Global Positioning System (GPS) data has shown transient crustal deformation in the Bungo channel, in southwestern Japan, since late August 2003, six years after a similar event in 1996--1997. Considering the southeastward motion at many of the GPS sites and the similarity between the 1996--1997 and 2003 events, it is highly likely that the transients were caused by aseismic interplate slip between the Philippine Sea plate and the Amurian plate. The 2003 aseismic slip is estimated beneath the Bungo channel and the relative slip increased with time on the west side, with a moment magnitude (Mw) amounting to 7.0. In contrast to the gradual subsidence of low-frequency earthquakes which began and ended in coincidence with the 2003 event, the 2003 transient crustal deformation proceeded at a roughly constant rate until the end of November 2003. The Bungo channel area may release energy accumulated from the subduction of the Philippine Sea plate by aseismic slip.