We have developed a seafloor acoustic ranging system as a possible future application to monitoring seafloor crustal movement with the DONET (Development of Dense Ocean-floor Network System for Earthquake and Tsunami) cabled observatory system. In 2007 we carried out an experiment for the seafloor acoustic ranging system. We deployed two precision acoustic transponders (PXPs) with about 750m spacing in Kumano-nada at a water depth of about 2035m. We collected 660 ranging data in this one-day experiment. The round-trip travel time shows a variation with peak-to-peak amplitude of about 25mm in the range. It was confirmed that most of the variation could be explained by the change in sound speed estimated from measured temperature and pressure. The remaining fluctuation in the acoustic measurements is ±2mm.