It is strongly desired to develop a cost-effective wind turbine system of floating-type in order to gain the renewable energy resources. In this study, the SPAR-type floating foundations using reinforced concrete are examined. The regular and irregular wave experiments are performed for 1/100 scale models of the prototypes. Three models are examined, that is, a uniform cylinder moored at the gravity center, a uniform cylinder moored at the upper position and a stepped-type floating cylinder moored at the upper position. The experimental results are compared with numerical predictions using potential theory and Morison's equation. The stepped-type floating cylinder moored at the upper position has shown smaller motions than the uniform cylinders.