The influence of carbon dioxide concentration on the making process of sake koji was examined. The mycelial growth was not affected by carbon dioxide concentration up to 10%. The acid carboxypeptidase production was stimurated by carbon dioxide concentration in the range of 2 to 7% but the production of a-amylase and glucoamylase was decreased by a supply of carbon dioxide. The supply of carbon dioxide affected the fatty acid composition, color of koji and components of koji extract. The volume of sake obtained and carbon dioxide evolution from moromi mash brewed by using koji cultured at different carbon dioxide concentration decreased with increasing carbon dioxide concentration. And the content of isoamyl acetate in fresh sake brewed by using koji cultured at different carbon dioxide concentration decreased with increasing carbon dioxide concentration.