The volume penalization vortex-in-cell (VP-VIC) method, which combines the vortex-in-cell and vortex penalization methods, is a kind of immersed boundary method that has the advantage of accurately calculating the vorticity field near the boundary. It is expected to be used as a design tool for shape optimization. In this study, the flow and performance of a small Savonius turbine rotor (rotor diameter DR = 245 mm) with S-shaped blades, which is used for small-scale distributed power generation with output less than 100 kW, were analyzed by numerical simulation. The results were compared with those of previous experimental studies to examine the usefulness of the VP-VIC method for fluid analysis around the Savonius turbine. The Reynolds number (??=?0??/?Re=U0DR/ν) based on the streamwise velocity U0 is 1.32×1051.32×105. The volume penalization vortex-in-cell (VP-VIC) method is applied for flow simulation, and the rotational motion of the rotor is analyzed using the calculation method previously proposed by the authors for the fluid force acting on the solid boundary (blade surface). As a result, the relationship between the rotational speed of the rotor and the output coefficient obtained in this simulation is similar in the experimental results of Golecha et al. [Appl. Energy 88, 3027–3217 (2011)] using S-shaped blades. In particular, with an increase in the load torque, the rotational speed of the rotor decreases, and the rotor stops momentarily. When the rotor stops, the positive torque of clockwise rotation acts on the advancing blade, while the negative torque of counterclockwise rotation acting on the returning blade decreases; hence, the rotor restarts immediately after stopping. In addition, it is established that the rotor output can be accurately predicted. Based on the above, it is reconfirmed that the VP-VIC method is beneficial for predicting the performance of a Savonius turbine.