For model experiments of nonlinear behaviors of a ship running in realistic ocean environment, it is necessary to generate short-crested irregular waves that have a specified and stationary directional spectrum in a model basin. However, the measurements of the waves generated by the double-summation method with side-wall reflections indicated that the variance of wave elevation is not stationary in space. Therefore, the authors carried out comprehensive numerical experiments to compare the double-summation method and single-summation one both with side-wall reflections. As a result, it is concluded that single-summation method even with side-wall reflections can be regarded as practically stationary in space but the double-summation method cannot.