The Equivalent Roughness Model (ER-model) based on shallow water theory is applied to investigate an interacted resistance force between wave and an obstacle. The model is modified to obtain an appropriate friction coefficient with an obstacle at the bottom by means of the ratio of area occupied by it on the grid area. Through a comparison with measured data, the quantitative verification about ER-model is carried out to discuss its applicability. From those results, it is indicated that the numerical error obtained around obstacles is affected by the physical modeling error such as vertical acceleration or local velocity that occurred due to the effect of obstacles rather than the truncation error. Furthermore, the relationship between friction coefficient and ratio of area occupied is discussed, and we could give the criterion for grid size selection for appropriate representation of the effect of obstacle in ER-model.