In order to improve the performance of an orthopter-type wind turbine, which is a paddle-type variable-pitch vertical axis wind turbine, the flow around the wind turbine with S-shaped blades and its performance were investigated by two-dimensional numerical simulations. The central angle of an S-shaped blade was varied φ = 60°, 90°, 120°, and 150°. The power coefficient of the wind turbine with a normal S-shaped blade is larger than that with an inverse S-shaped blade. The maximum power coefficient of the wind turbine with the normal S-shaped blade for φ = 90° and 120° increases more than that of the other blade. This increase of power coefficient is caused by increasing rotation torque due to the generation of a vortex in the outer concave side of S-shaped blade near azimuth angle of θ = 120°, and increasing revolution torque due to the generation of three vortices behind the S-shaped blade near an azimuth angle of θ = 240°