To improve the quality of mirin, various molds were screened for mutants with high acid carboxypeptidase (ACPase) by the method of ultraviolet radiation. Mutants, Aspergillus oryzae 9-12 and Aspergillus shiroussamii 6082-60 showed activities of ACPase about 2~6 times higher than their parent strains. Aspergillus oryzae 9-12 and Aspergillus shirosamii 6082-60 were the most suitable strains for preparing koji in mirin by the conventional or improved methods. The results showed that total sugar, reducing sugar and total nitrogen were almost the same values in mirin prepared by both methods. The yield of mirin was higher in the improved method than in the conventional method. The clouding formation of mirin appeared in the conventional method ; however, mirin prepared with the mutant koji by the improved method did not show clouding formation.