The optimum arrangement of the groyn has been determined mainly by hydraulic model experiments. This method has a weak point not to be able to determine groin arrangement to river courses with arbitrary alignment. At the atandpoint that the numerical model and the hydraulic model test are complemental to each other, three dimensional numerical model was developed to decide the optimal arrangement of the groin This model includes the influence of forces exerted by groins on the flow of rivers. The result of calculation corresponded so much to that of existing experiments with regard to the state of the three dimensional flow and the river bed change, and also this model enabled us to decide the optimum arrangement of the groin to the river with arbitrary alignment.