This paper investigates the statistical measurement of income distribution and evaluates the income redistribution effect of income tax system in Japan, using National Survey of Family Income and Expenditure 1984-2004. This paper finds first that income inequality has been widening over time, especially in the young (age 39 or less) and middle (age 40-59) groups, second that tax role of income redistribution has been weakened since 1984, and third that the income redistribution effect by taxation is strong in the old (age 60 or above) group because before tax distribution of income is already very unequal among them.
