The output power coefficient of the Savonius rotor should be improved for better practical applications. So far, new Savonius rotor has been developed to improve the minimum output coefficient by adding semi-elliptical blade. Thus, the purpose of this research is to investigate the influence of the additional semi-elliptical blade’s position on the output coefficient. Flow around the rotor was simulated by using the regularized lattice Boltzmann method. The virtual flux method was used to express the shape of the rotor on a Cartesian grid, and the multi-block method was used for local fine grids of the rotor. The rotation speed of the Savonius rotor was maintained constant, and its performance was evaluated by the output power and torque coefficients. As a result, the semi-elliptical blade successfully generated additional positive torque in the range of the advancing phase and improved the minimum output power coefficient of the rotor during a cycle. When the moment arm is short, the semi-elliptical blade did not generate large negative torque in the range of the returning phase owing to its position behind the main blade in the wind flow direction. The output power coefficient of the new Savonius rotor was improved compared to that of the traditional one depending on the length of the semi-elliptical blade’s moment arm.