The new type of shrouded wind turbine called "wind-lens turbine" has been developed. The wind-lens turbine has a brimmed diffuser called "wind-lens", by which the wind concentration on the turbine blade and the significant enhancement of the turbine output can be achieved. A simultaneous optimization method for the aerodynamic design of rotor blade and wind-lens has been developed. The present optimal design method is based on a genetic algorithm (GA) which enables multi objective aerodynamic optimization. In the present study, aerodynamic performances and flow fields of the Pareto optimal solutions of wind-lens turbines designed by the present optimal design method have been investigated by windtunnel tests and three-dimensional Reynolds averaged Navier-Stokes (RANS) analyses. Output power coefficients obtained from the wind-tunnel tests in the optimal wind-lens turbine exceeded the Betz limit, which is the performance limitation for bare wind turbines. The numerical results and the experimental results show that the suppression of flow separations in the diffuser is important to achieve significant improvement in aerodynamic performances. 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.