At the docking of a three-dimensional moving vehicle to a fixed base in deep water, an automatic system is indispensable to assure reliable and safe operation. In the paper, an automatic docking system using ropes is proposed for such situation, and basic concept and characteristics of the system are presented with some results of feasibility study of the system. In the system, ropes fixed to the base are connected to the vehicle by sonar trackers launched from the base, and then, the vehicle is docked to the base by the ropes controlled by a central controller in the base. Real time estimation of state variables of the vehicle, i. e. position, velocity and attitude of the vehicle, from the rope lengths, and the control of the vehicle by the ropes based on these information are essential in the system. To investigate practical feasibility of the system, especially those of real time state estimation and the vehicle control, simulation study is made using a small size mini-computer which can be installed in such underwater base. From the simulation results, it is shown that a near optimum filter can be effectively utilized to estimate the state variables of the moving vehicle, with which the vehicle can be controlled by the ropes with sufficient accuracy.