The FIT obligates the power transmission and distribution companies to procure renewable energy for a fixed period at a fixed price. However, the public burden due to cost of purchase has become heavier. Furthermore, since there are suppliers who do not operate immediately after FIT certification, the introduction of renewable energy is being delayed. In this document, we aim to balance reducing the public burden and increasing the amount of renewable energy, by using real option analysis to propose an optimum certification period and purchase price.