A brewing yeast mutant with perfect sugar fermentation capacity was isolated by mutagenizing the Saccharomyces pastorianus transformant, which carries an integrated glucoamylase gene and has one copy of non-functional alpha-acetolactate synthase gene. The mutant was able to utilize maltotriose efficiently, and the maltotriose fermentability in YNB-2% maltotriose medium increased from 32.4% to 72.0% after 5 d in shaking culture. The wort fermentation test confirmed that the sugar fermentation property of the mutant was greatly improved, while its brewing performances were analogous to that of the wild-type strain and the characteristic trait of shortened beer maturation period was retained. Therefore, we believe that the brewing yeast mutant would benefit the beer industry and would be useful for low caloric beer production.