A three dimensional attitude control system for a streamlined cruising type autonomous vehicle are studied. A streamlined vehicle are appropriate for deep sea investigation in a wide area for its high efficiency of energy consumption, converting potential energy to velocity energy. Authors studied a simple altitude control system for such autonomous vehicle, based on distance observation by four ultrasonic fan beams to differeat directions. The system is based on two dimensional pitch control, and if it is applied to this type of vehicle, it is shown that interference between yaw and pitch motion causes serious velocity decrease by large sideslip angles due to small lift in the side direction. In the paper, effective three dimensional attitude control method based on so-called Bank-to-turn control, which is named. Roll-Pitch-Roll control in the paper, is proposed. And the system performance is investigated by simulation study for various cases, from which the following results are obtained. (1) For large yaw angle rotation, RPR control improves rotating speed remarkably without decreasing cruising velocity, which can be hardly realized when each axis is controlled independently due to the motion interference. (2) Improvement of speed decrease for large rotation angles both in yaw and pitch direction. From these results, it is concluded that the proposed system is utilizing strong lift force in pitch direction effectively, and appropriate for attitude control for this energy saving type cruising vehicle with asymmetric shape.