A Wells turbine operating under an irregular wave condition has been analyzed theoretically. It is shown that the turbine performance depends on a unique parameter, which includes characteristic parameters of irregular waves, turbine speed and dimensions of the turbine and air chamber. In order to obtain the optimum value of this parameter, a model test has been performed in a computer-controlled wind tunnel, which can simulate any kind of oscillating flows based on spectra of irregular waves. From this value, a set of optimum dimensions of the Wells turbine system can be determined. Furthermore, starting and running characteristics of the Wells turbine have been obtained by a computer simulation and compared with the experimental results. It is possible to predict the optimum value of the parameter and behavior of output coefficient by computer simulation.