We try to solve the problem of efficiency drops on Savonius wind turbine at higher tip speed ratios using twisted buckets, which are expected to increase drag acting on the advancing bucket and to decrease drag acting on the returning bucket. These effects may be given by three-dimensional flow structure due to twisted buckets. Flow visualization were performed around four types of twisted Savonius wind turbine models and velocity distributions were obtained by PIV. Phase-averaged velocity field in the rotating angle range of 0° to 180° was used to reconstruct three-dimensional velocity distribution considering geometric symmetry of the turbine and periodicity of flow structures appearing in each phase of the turbine. Pressure distribution was then estimated by solving Poisson equation of pressure from the PIV data. From the result, two major effects of twisted buckets were clarified; one is modification of the drag force on each bucket, and the other is interaction with effect of the gap between two buckets producing a function to reduce drag on the returning bucket.