This paper introduces the methodological challenge of identifying and quantifying the interdependencies among several critical infrastructures. First, interdependency structures during a natural disaster are modeled based on past events, considering supply (electricity, water, and gas), communication (internet and telephone), and transportation infrastructures (road networks). Interdependencies are defined with respect to physical, functional, and socio-economic interrelationships. A quantification strategy is then introduced based on empirical surveys and economic models. As a case study, the developed model is applied to the 2004 Mid-Niigata earthquake, which severely damaged infrastructure systems in the northern mountainous region of Japan.