Marine vessels require periodic cleaning treatment to maintain the hydrodynamics and fuel efficiency that can be affected by marine organisms growing on the hull surface. Traditional method to ship hull cleaning is costly due to amount of the availability and risk. Underwater robots are being developed worldwide as another method to the task with reduced such burdens. A Remotely Operated Vehicle (ROV) for ship hull cleaning has been on development in Ishii laboratory, Kyusyu Institute of Technology, Japan. This study introduces a center of gravity control mechanism that enables the ROV align itself to curved surfaces of ship hulls by controlling its center of gravity.