| This study proposes a method to obtain the social marginal costs of public funds (SMCF) that allows for heterogeneity on a household basis as well as labor supply responses along both the extensive and the intensive margins. To demonstrate our methodology, we take the example of the 1999 national income tax reform in Japan and evaluate it by estimating the SMCFs for changing marginal tax rates in different income brackets. We estimate the discrete choice model (DCM) of labor supply using a 1997 data set of Japanese households, and we use the estimates to generate the SMCFs with a DCM micro-simulation. We evaluate the simulated SMCFs with various distributional weights and find that the value of the SMCF for a 1% increase in the marginal tax rate in any given income bracket decreases as we move across brackets from the bottom to the top. This finding suggests that the national government should have made the Japanese income tax system more progressive rather than less progressive as carried out in the 1999 reform. Our method is readily transferrable to tax reforms in other countries as well.