Sperm whales are known to dive to a depth of over two thousand meters and it is also known that they emit loud impulsive broadband sounds called "clicks" during their diving. But little is known about sperm whales' behavior because of a shortage of methods to observe them during their dives. Our attempt is to use autonomous the underwater vehicle (AUV) as an observation platform for sperm whales. The tasks of AUV are to recognize sperm whales individually and track them in real time by listening to sperm whale's clicks with 4-hydrophone array and detecting its direction. As the initial stage for these objectives, we set up an experiment for our observation system using a boat instead of AUV and a support boat. In the middle of August 2003, we carried out an experiment to test our observation system off Ogasawara Islands. Two boats each with a hydrophone array were deployed mainly in the west or southeast of Chichijima Island in ocean depth of about 1000 meters. During the experiments over five days we encountered at least fourteen sperm whales. The result of this experiment indicates our observation system is suitable for tracking of sperm whales in sub-real time. We also acquired sufficient sound data of sperm whale's vocalization for off-line verification. The analysis of the results is presently underway. This experiment can be considered as the preliminary step to apply this kind of technique to develop innovative observation for underwater ecosystem system using AUVs.