<p>The goal of this study was to construct a multi-legged robot to provide stable operation on the seabed. It would be difficult to reach the work destination only by walking movement, since the ocean floor is often an irregular area owing to the rocks of various sizes and also there are many obstacles such as marine resources in the shallow area. We propose a remotely operated vehicle (ROV) that avoids obstacles on the seabed by swimming locomotion. In this paper, we implemented an algorithm to control the posture control and the output direction of thrust by moving the center of gravity (COG) of a radially arranged six-legged octopus-motion-like ROV (RASOL), and demonstrated the swimming locomotion in a three-dimensional manner. As a result, RASOL could swim by flapping its legs and move toward the goal in a three-dimensional manner by controlling the position of the COG.</p>