This report describes an automatic installation system of offshore structures such as breakwater caissons which has been developed for the purpose of reducing labor during caisson installation work and the cost of construction, maintaining the safety of workers, improving construction efficiencies and so on. The system consists of an Engineering Work Station (EWS) with A/D and D/A converters, winches operated by controlsignals from the EWS and an observation system used to measure wave height, caisson motion and so on, and it enables automatic construction of offshore breakwaters. As a caisson is installed, the system obtains date on caisson position and motion, wave height, and wire tension and displacement through the observation system in order to predict the motions of the caisson by use of the EWS. Control signals are then output from the EWS to each of the winches for causing the caisson to move to the optimum position for installation. This procedure is repeated, and the caisson is finally installed at the target position near the side of a breakwater. By use of the automatic installation system,the present winch operation which is directly hand-operated by several operators on a floating crane is able to be automated,and the working conditions on offshore constructions are expected to be improved. The EWS includes a program allowing installation work to be performed using two associated units. One is a tugboat support system and the other is an anchor wire system. In this paper, hydraulic model experiments for the tugboat support system is reported, and the validity of the control algorithm of the automatic installation system is confirmed. It is then certified from the experimental results that this system is able to automatically install breakwater caissons with good accuracy.