A robot system includes a plurality of segments 3, joints 4 for linking the segments together, drive units 5 for actuating the joints, and a controller 8 for controlling the drive units. Further, the robot system has bladders 2 filled with a fluid being of lower specific gravity than the outside environment. A center of buoyancy differs from a center of gravity, and the robot system has a specific gravity of greater than 1 relative the outside environment.