The reason why carbamide was not detected in mirin made with koji prepared with A. oryzae but was in mirin made with koji prepared with R. oryzae was studied by using the conventional mirin mash in a laboratory-scale. The largest amount of carbamide present in the beginning of the mash became undetected in 16 days for koji prepared with A. oryzae . However, about 30% of carbamide remained in 60 days for koji prepared with R. oryzae , suggesting that A. oryzae has acidic urease activity as well as neutral one. Thus, A. oryzae IFO 5238 out of 63 strains, grown in the broth, was selected for decomposing 44.8% of carbamide at pH 4.O. Its urease activity was mainly in mycelia but not in the broth. Subsequently a method to reduce carbamide content in mirin mash was developed by mixing two kinds of koji : the one prepared with A. oryzae IFO 5238 and the other propared with P. oryzae LC, a mutant of R. oryzae IFO 4719, forming larger amount of glycerine and Smaller amount of carbamide. The optimum ratio of koji amount was determined: 0.3 koji prepared with A. oryzae IFO 5238 with respect to 1.0 koji prepared with R. oryzae LC. Next, two kinds of mirin were made and their qualities were compared: the one with the mixture of the two kinds of koji described above and the other with R. oryzae LC koji alone. Carbamide was not detected in the former mirin but was in the latter, while the rest of their composition was comparable. The results indicate that the method makes manufacturing of mirin containing larger amount of glycerine and no carbamide possible.