Happoshu (<25% malt ratio in the raw material) had more than a 25% share of the Japanese beer market in 2000. The flavor of Happoshu is similar to that of Japanese adjunct beer, but the amount of the components are different, e.g., polyphenols. We have investigated to improve the flavor stability of Happoshu. By repeating the olganoleptic tests and measurements of carbonyl compounds using GC and HPLC, we selected some indices such as 3-methyl-butanal, which correlated with the stale and papery flavor of Happoshu after storage. These indices agreed more with the flavor stability of Happoshu than did the carbonyl compounds such as diacetyl. An index, 3-methyl-butanal is derived from malt and reduced by yeast during fermentation, while this compound is also produced as an intermediate of isoamyl alcohol production from sugars or leucine by yeast. During the fermentation of Happoshu, 3-methyl-butanal was partly reduced by yeast on the first day and then slightly increased, and again decreased during the secondary fermentation. Based on this compound's behavior, we considered that the reducing activities of yeast and the cell numbers at the end of the primary fermentation (a beer transfer stage) might be important for the flavor stability of Happoshu. In order to investigate the criteria of yeast selection, we tested the fermentation performance of yeast strains. The carbonyl production and reduction by yeast were varied with four yeast strains and did not agreed with the results of Acidification Power Test. Furthermore, the yeasts suitable for fermenting a standard beer wort were not always suitable for the Happoshu production. The non-flocculent character of yeast was an effective factor for reduction of the carbonyl compounds in Happoshu. Using 30L fermentation tanks, we tested the flavor stability of Happoshu using six strains derived from a certain clone of a bottom-fermenting yeast. The concentration of 3-methyl-butanal inversely correlated with the cell numbers at the end of the primary fermentation. As we expected, the Happoshu samples produced by the yeast strains, which reduced more carbonyl compounds (3-methyl-butanal, phenylacetaldehyde, etc.), had a better flavor stability after storage. The sulfite levels due to the yeasts (3.6 - 6.7 ppm) were not related to the staling degrees in this experiment. The fermentation process is known to be important for producing beers with good flavor stability, i.e., the yeast physiology is related to the production of sulfite, which prevents the staling reactions of beer. Beer stale flavor and the degree of contributing factors for flavor stability vary with the type of beer. For the flavor stability of Happoshu, the fermentation process is found especially important. One of the mechanisms should be that yeast plays an important role in decreasing the amount of stale compounds and their precursors.