The present invention relates to a flavor composition or fragrance composition which can satisfy diversified requirements for flavored products, as well as to a flavor-improving agent which can improve the quality and release of aroma of a beverage or food. More particularly, the present invention relates to a flavor composition or fragrance composition which comprises an optically active S-alkyl 2-methylbutanethioate as an active ingredient, a flavor- or fragrance-added product, a flavor-improving agent which comprises an optically active S-alkyl 2-methylbutanethioate as an active ingredient, and a beverage or food having an improved flavor. The optically active S-alkyl 2-methylbutanethioate includes S-alkyl (R)-2-methylbutanethioate and S-alkyl (S)-2-methylbutanethioate.