In the offshore nourishment, whether sand supplied to the offshore zone is transported shoreward by wave action and accumulates to form a foreshore is important in enhancing the shore protection effect of sand nourishment. A model for predicting such beach changes is developed here using the contour line change model and taking the grain size effect of the nourishment sand into consideration. This model can be used to predict the three-dimensional changes of a sand body artificially supplied to the offshore zone. To verify the effectiveness of the model, tracer tests using colored sand were carried out, and the diffusion pattern of sand was compared with the results of numerical simulation. The model was also applied to Kujukuri beach, where detached breakwaters have been built.