Recently, demands for increased safe operation and ocean environment protection, especially preventing oil pollution in the sea are remarkable as the international tendency. On the other hand, the trend of the shipping world indicates clearly a strong demands to reduce the cost for operations and maintenance of ships. From the viewpoint of survey and inspection of ship hull structure, the improvement of reliability of inspection, safety of work and reduction of costs for inspections are one of the most required subjects today. In order to correspond to the social requirements for surveys and inspections of hull structures, the authors are studying and developing the new monitoring system by the underwater RTV robot (MITSUI RTV-SHIP). The advantages of the RTV-SHIP are as follows : 1) All the tank walls can be inspected and easily recorded on video tapes. 2) All the operations can be controlled on the upper deck by minimum operators. 3) All the surveys can be performed on voyage. The basic tests of RTV-SHIP with regard to positioning in the tank, measurement of plate thickness and large deflection of panels were carryed out and satisfactory results were obtained. The utility of the RTV-SHIP was proved in this study and further tests for the actual ships are now planned with the aim of realizing this system. The RTV-SHIP is expected to make a major contribution to the safe operations and ocean environment protection.