Number of offshore wind farms is still increasing all over the world. The offshore wind power is greater than that on land. In addition there is no restriction of site and road for construction. However the offshore wind farm is expensive. Therefore the capacity in electricity per a wind turbine needs to be much larger than those on land and onshore. Especially, in Japan, there is so little shoal that not traditional fixed-bottom wind turbines but floating wind turbines are required. So the capacity in electricity per a single turbine for offshore wind farms should be larger than 10 MW or 15 MW. Furthermore the most troubles in the present wind turbine systems occur in the multiplying gear. Hence the trend of the development of wind turbine systems is the direct drive without gear. In case of normal conductive generators which are composed of copper windings and iron core and teeth, the weight of generators becomes huge. In this study we studied the optimum electrical design of 15 MW fully superconducting generators with REBCO superconducting tapes. We first investigated the electromagnetic properties of currently developed REBCO superconducting tapes. To reduce the ac loss in REBCO superconducting tapes, we intend to adopt the ac loss reduction technique which was applied to construct a 3phase-66/6.9kV-2MVA superconducting transformer. It is the combination of scribing and special winding. Using the observed and theoretically estimated properties, we conceptually designed 15 MW fully superconducting generators with the magnetic field at the gap and the operating temperature as a parameter. The diameter of superconducting rotor was supposed as 4.5 m. For each case we investigated the weight of generator, the required length of REBCO superconducting tapes, ac loss of the tape, efficiency and the construction cost by numerical simulation using the software on the market.