Offshore floating-type wind turbines are less affected by water depth and are expected to be an effective power source for sustainable development. An experimental study was begun to realize a practical offshore floating-type wind turbine in the ocean near the Goto Islands, Nagasaki. The most striking characteristic of offshore floating-type wind turbines compared with traditional foundation-mounted wind turbines is that they pitch and roll with the waves. Research on the sea current and wave forces acting on an offshore floating-type wind turbine is important for launching the development of this wind turbine technology in the waters of Japan. The result of this study is important for installation and actual operation. The forces acting on the wind turbine were estimated through a model experiment and numerical simulation. Thirty-seven observation points from the Nationwide Ocean Wave Information Network for Ports and Harbours in Japan were taken as estimation sites.