As climate change adaptation is becoming a recognized policy issue, the needs are growing about quantitative economic evaluation of adaptation-related public investments, particularly in the context of climate finance. Irrigation, which enhances and stabilizes water supply for farming, is a potential means of climate change adaptation, but attempts of economic evaluation of its effectiveness as an adaptation measure are few in part because such assessment requires an integration of various types of simulation analyses. Against this background, we conduct a case study of a Kenyan irrigation development project (Mwea Irrigation Development Project), which is undertaken by the Kenyan government with a loan from the Japan International Cooperation Agency (JICA), to evaluate the effectiveness of the project for climate change adaptation by using a combination of simulation models. Specifically, we conduct a simple downscaling of CMIP5 climate simulation data, whose outputs are fed into a hydrological model (SHER model) and a yield forecasting model (DSSAT model). With these simulation data, together with data of socioeconomic parameters drawn from existing and original surveys, we compute economic variables such as farmers’ average income. Climate and other uncertainties are incorporated into analysis without probabilistic weights (conforming to the Robust Decision Making approach, e.g., Lempert et al., 2013) to highlight vulnerabilities for local farming. The results show that despite uncertainties of precipitation trends, increased temperatures due to climate change have a general tendency to reduce rice yields, and that irrigation development will mitigate that income impacts from the yield loss, i.e., it will likely be effective as a means for climate change adaptation.