Security management cost for securing information is increasing rapidly, as increasing the use of electronic information. In this paper, the authors focused on the two respects. The one is security level lifecycle of the Sensitive Information (SI). The other is SI that has characteristics to decrease security level over time. They proposed the method that total security management cost reducing than before applying by differential costing over security level lifecycle. Also, they considered of predictability on security level decrease together. Last, they expressed the cost reduction target area through a comparison of the cost model.