This study examines how changes in the rate structure, deductions, and tax credits for personal income tax affect the inequality of posttax income, using micro data on Japanese households. The main contribution is the use of the fixed-income approach to assess the decomposed redistributive effects of the personal income tax. Several insights are obtained. First, the redistributive effects of the Japanese income tax declined during 1984–2009. Second, base effects substantially exceeded rate effects over this period. Third, credit effects greatly decreased the redistributive effect, as Japanese tax credits were proportional to tax liabilities with upper limits.