This paper describes that an alternative fuel production system provided by offshore wind energy is technically and economically feasible. The following knowledge was acquired as a result of studying element technologies aiming at an initial design of an alternative fuel production system using floating wind power. (1) The sea area where the installation of the wind farm was promising was clarified offshore in Japan. And the energy potential was estimated. The amount of the electric power is three times or more the amount that aims by 2010 (2) The box-girder type floating structure mounted two 5MW wind turbines was proposed as a concept. And it was clarified that the structural safety can be kept in a offshore storm set as a design condition. (3) The conceptual design of a new wind turbine which lowered the center of gravity and is suitable for floating body was carried out, and the effect of inertia force was confirmed. (4) EPR of the system was shown to be about 5 or 6 in the evaluation of the energy balance. (5) It was clarified that the maintenance cost plays an important role in the economic evaluation.
