Authors are now developing a streamlined cruising type autonomous submersible controlled by rudders and elevators, which is intended to investigate surface of the sea bed continuously for scientific observation as a research project in our university. A streamlined vehicle is appropriate for wide area investigation for its high efficiency of energy consumption, converting potential energy to velocity energy. In order to collect underwater information continuously taking pictures, it 12 required to keep low altitude from the surface on the condition that it keeps safety distance. To realize such missions with sufficient reliability, a simple altitude control system based on distances to the surface observed by several narrow ultra sonic fan beams to different directions is proposed, and availability of such system for the height control is evaluated by simulation study. From the study, it in assured that the proposed control system can keep the predetermined altitude for most cases within 20% margin. and basic feasibility of the system is verified.