We have developed a seafloor acoustic ranging system to monitor local seafloor crustal deformation. In August 2007 we carried out an experiment to estimate the long-term stability of acoustic measurements collected by the system. We deployed four precision acoustic transponders with about 920m and 590m spacing at the depth of 2035m in Kumano-nada, southeast coast Japan, and we collected acoustic ranges for 4months. The observed round-trip travel times showed a variation with peak-to-peak amplitude of about 0.75m in the range. It was confirmed that most of the variation could be explained by changes in sound speed. The standard deviation in the acoustic measurements was about 1.0cm after the correction for sound speed, and the average value of the ranges remained constant within 1cm.