Many industrialized countries have concerned the increasing health inequalities. Alleviating the inequalities is a common challenge in the international community. Especially, in the United States, researchers have held a warning for the country’s rapidly widening health inequality. Japan, previously recognized as a relatively egalitarian country, also currently suffers increased social disparities, potentially leading to socioeconomic-based health inequalities. On the other hand, the effects of individual income on health have been well established in western countries but there are continuing discussions about the impacts of income distribution (in other words, regional income inequalities or relative income) on health. Its pathways have also been discussed vigorously. The purpose of this study is, by conducting empirical studies of the US-Japan comparisons, to obtain answers to the following questions: how much social inequalities adversely affect our health?; what is the mechanisms of them?; and how much health inequality can be reduced by the alleviation of a certain amount of social inequality? In this study, we will use Japan’s and the US’s samples from major national representative surveys including information pertaining socioeconomic and health variables. We will also utilize the longitudinal panel data from the Health and Retirement Study in the both countries, two Japanese longitudinal data managed by the research projects I belong to, and several aggregated regional data reported by the governments. As a main statistical method, we will use a multilevel analysis which can validly model the relationships between variables in different levels such as individual (health), region (inequality), and country (social characteristics). We will set regional income inequality measures and social capital as predictors of health outcomes such as perceived health, health risk behaviors, mortality, morbidity, and disability onset. In longitudinal analysis, we will model temporal changes in regional inequality as a predictor of mortality or morbidity using survival analysis, enabling us to figure out the causal relationships between them. We will evaluate major mechanism hypotheses such as social stratification, social comparison, and social capital by comparing different models using various indices of income inequalities as well as different area units to calculate the indices. By the comparisons between the two typical industrialized countries, we can figure out the effects of inequality on health under the consideration of the differences in social characteristics such as culture and social security systems. We believe that the results of this study provide important suggestions in health policy among industrialized countries including the United States and Japan.