This paper discusses an evaluation problem of the benefit of reservoir under uncertainty. A reservoir has a function of flow stabilization. Since natural stream flow is uncertain, the available water in stream sometimes becomes shorter than water demands and then a drought happens. Constructing a reservoir makes the stream flow stabilized and probability of drought is reduced. We should consider how to evaluate the benefit of stream flow stabilization or reduction of drought probability. In the field of welfare economics, option price and expected consumer surplus are proposed for estimation of benefit of a project in a risky world. These measures are formulated and examined. The focus of this paper is to find the appropriate way to measuring the benefit of a reservoir through examination of these measures.