In 2003, the Japanese Diet passed legislation to abolish part of the special deduction for spouses (Haigusha Tokubetsu Kojo) which would take effect in the 2004 tax year. This paper investigates the impact of this tax reform on female labour supply in Japan. A number of earlier studies have shown that married women are likely to adjust their labour supply in order for their husbands to become eligible to take advantage of this deduction. By using the first and second waves of the Keio Household Panel Survey (KHPS) collected in 2004 and 2005, this paper uses the difference-in-differences approach to examine whether there has been any change in the behaviour of female labour supply as a result of this tax reform. The empirical evidence indicates that the tax reform in 2004 had a negative and statistically significant impact on both the decision of female spouses to enter the labour market and on the number of hours they worked conditional on their participation in the labour market. In addition, the empirical evidence obtained does not support the Douglas-Arisawa Law.