Beach has been severely eroded on Ajigaura Beach in Ibaraki Prefecture. As a measure, a sand back pass method was adopted, by which 10,000 m³ of sand is excavated from the north end of the beach a year and sand is transported upcoast for beach nourishment. However, nourishment sand was quickly transported to the original location while losing the effect of the nourishment. This method requires a large cost for transportation of sand, so the continuation of this method is difficult. Instead of this method, a method to create a stable beach by constructing an impermeable detached breakwater was considered by the numerical simulation using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept).