In situ modification ((R,R)-tartaric acid and NaBr were directly added to the reaction media) of fine Ni powder (FNiP) and reduced Ni catalysts was applied to the enantio-differentiating hydrogenation of methyl acetoacetate (MAA). The high optical yield of 89% was attained by the reduced Ni catalyst. This is the highest value reported so far for the hydrogenation of methyl acetoacetate using the in situ modification. The addition of a small amount of NaBr to the reaction media increased both the optical yield and hydrogenation rate, while the hydrogenation rate decreased with the addition of NaBr to the modification solution for the conventional modification method. NaBr added to the reaction media for the in situ modification would have both of the following roles: (i) Na+ increased the optical yield and the hydrogenation rate; and (ii) Br- increased the optical yield and decreased the hydrogenation rate.