In the various investigations of the port area and the construction management of the port facilities, it is very important to observe underwater states, to monitor operations, and to confirm an execution by viewing. These work are carried out mainly by divers and partly by ROVs at present.
　On the project of the improvement of Tokyo Bay Waterway, it is needed to observe and take a picture to confirm the effect of the artificial fish reeves constructed by the blocks of the ruined Third Fort of Tokyo Bay and to maintain the function of the waterway in the port area.However, in this sea area, the current is fast and the water depth is too deep for diver's work.Therefore, it is needed to research and develop a new underwater observation system in order to carry out viewing work safely and efficiently.
　We studied the system suitable for this mission in the first step. Consequently, we proposed a new underwater observation system using Autonomous Towed Vehicle and decided the functions and specifications of the total system to perform the given mission. This system can be used as a towed mode, an autonomous mode, and a kite mode according to the current speed.
　We investigated the stability of the vehicle by the basic motion simulation in the next step.As a result, it is clear that an additional weight and an enlargement of forewing's area are required. Moreover, the feedback control of heaving and pitching by parallel-motion of forewings and feedback control of rolling by differential-motion of forewings are needed, too.