We have developed an observation system for seafloor crustal deformation measurement combining the two following techniques: the precise acoustic ranging and kinematic GPS (Global Positioning System) positioning. On July 14, 2003, we installed the units for this observation system at the seafloor of the Kumano Basin, Japan (33.58¿N, 136.60¿E). We performed repeated observation six times on 1) July 15--16 and 21, 2004, 2) November 9--10, 2004, 3) May 18 and 20, 2005, 4) July 19--20, 2005, 5) August 18--19, 2005, and 6) August 29--30, 2005. The present observation system can stably determine the seafloor position with an RMS repeatability of ¿3 cm in both horizontal components. Two M7 class offshore intraplate earthquakes occurred only 60 km from the Kumano Basin site on September 5, 2004. We observed the coseismic horizontal displacement of 18 cm at the Kumano Basin site using the presently developed seafloor observation system. Our observation system can provide seafloor crustal deformation associated with crustal activities in offshore areas.