Investment planning for power plants involves making a long-term plan covering various facility investments, such as the construction of a new power plant or the replacement of an old plant with a new one, under uncertain environments. In this paper, we propose an optimization method for such a planning problem. Our method is based on decision tree analysis, in which uncertain environments are described as scenarios. The maintenance of existing plants is also taken into account by introducing the option of large-scale repair in addition to replacement. In order to avoid combinatorial explosion of failure event scenarios of plants, we introduce the concept of failure risk cost which contributes to simplification of the decision tree and reduction of the calculation time.