An optimum aerodynamic design method for the new type of wind turbine called "wind-lens turbine" has been developed. The wind-lens turbine has a diffuser with brim called "wind-lens", by which the wind concentration on the turbine rotor and the significant enhancement of the turbine output can be achieved. In order to design efficient wind-lens turbines, an aerodynamic design method for the simultaneous optimization of rotor blade and wind-lens has been developed. The present optimum design method is based on a genetic algorithm (GA) and a quasi-three-dimensional design of turbine rotor. In the GA procedure, the Non-dominated Sorting Genetic Algorithm II (NSGA-II) is used as evaluation and selection model. The Real-coded Ensemble Crossover (REX) is used as crossover model. The quasi-three-dimensional design consists of two parts: meridional viscous flow calculation and two-dimensional blade element design. In the meridional viscous flow calculation, an axisymmetric viscous flow is numerically analyzed on a meridional plane to determine the wind flow rate through the wind-lens and the spanwise distribution of the rotor inlet flow. In the two-dimensional rotor blade element design, the turbine rotor blade profile is determined by a one-dimensional through flow modeling for the wind-lens turbine and a two-dimensional blade element theory based on the momentum theorem of the ducted turbine. Total performances and three-dimensional flow fields of the optimized wind-lens turbines have been investigated by Reynolds averaged Navier-Stokes (RANS) simulations, in order to verify the present design method. The RANS simulations and the flow visualization have been applied to conventional and optimum design cases of the wind-lens turbine, in order to elucidate the relation between their aerodynamic performances and the flow fields around them. The numerical results show that separation vortices behind the wind-lens brim play a major role in the wind concentration and the diffuser performance of the wind-lens. As a result, it is found that the aerodynamic performance of wind-lens turbine is significantly affected by the interrelationship between the internal and external flow fields around the wind-lens.