The purpose of this paper is to examine the erosion countermeasures which were devoted a large amount of the financial resources and to discuss the theoretical measures of the erosion problem on Ishigaki Island. First of all, to examine the erosion countermeasures in the past, we analyzed the field data of the island from the point of view of cost-benefit. Next, we discussed the theoretical measures in terms of the cost burden involved to achieve erosion protection. As a result, we clarified that the cost benefit of each countermeasure differs greatly and that some erosion countermeasures have been practiced in low risk areas. In order to achieve effective erosion countermeasures, farmers should practice the most cost-effective means selected from the high risk fields. In order for less soil to be eroded away, they should spread the countermeasures from the high risk fields to the low risk ones. So as to accomplish our policy objective, we need to insist that farmers have to practice the erosion countermeasures in the fields which need them. And the reference level divides the cost burden between farmers and government.