This paper deals with a case study on the Shiwha Coastal Development Project in terms of cost-benefit analysis. The project had carried out for 20 years (1987-2006) in attempting to reclaim the Shiwha wetland area surrounded by shallow sea waters. Many economists and environmentalists argued that this huge project would result in marine environmental damage serious enough to offset the targeted economic benefit. Consequently, there have been many attempts to evaluate this project economically. In this study, the major costs include construction costs and loss of living marine products while the major benefits come from the reclaimed land for the purposes of agriculture, industrial complex, and residence. Since the technological progress over time is assumed to reduce the costs associated with this project, some what-if analyses were performed in terms of different development time and purpose. The primary conclusions resulting from this cost-benefit analysis on the Shiwha Coastal Development Project are two important elements. 1: It is not so good idea to launch the huge coastal development project like the case of Shiwha area without considering the irreparable damage of living marine resources. As a result, the investor is required to take into account the marine environmental protection technologies that allow reducing the relevant costs as well as increasing the economic benefits. 2: In performing the huge coastal development project, the investor also needs to make proper decision on the development scale and purposes. This case analysis reveals that multi-purpose development approach is better than single-purpose one economically. Finally the factor of scale merit should be taken into consideration in carrying out the gigantic coastal development project in view of demand for and supply of the reclaimed land. Key Words: Cost-Benefit Analysis, Coastal Development, Technological Progress, Environmental Protection, Development Scale and Purposes.