In order to improve the quality assurance of the disaster mitigation projects, the economic effect of these projects in the hazardous areas was analysed. Eight project sites were selected for analyses based on the disaster data during the previous 10 years, and the investment effect was evaluated using a benefit cost ratio (B/C). The benefit was estimated using the historical disaster data and presumed to continue for 30 years, while the cost was assumed with the total project cost. Analysis results indicate the B/C ratio is larger than 1 in the difference range, depending on factors such as impact areas and discount rates. According to the analysis results, the average B/C of the eight projects is 4.1 with assuming the discount rate of 4% and the impact diameter of 5 km, which implies that a disaster management project in hazardous areas will give the positive investment effects.