Provided is a method for improving the optical purity of an optically active 2-hydroxycarboxylic acid useful as a raw material for producing pharmaceuticals, agricultural chemicals and industrial products. The hydroxycarboxylic acid represented by the following general formula (Ia) or (Ib) is reacted with at least one non-optically active base selected from the group consisting of alkali metal alkoxides and secondary amines in the presence of a solvent. And recrystallizing to obtain a hydroxycarboxylic acid salt represented by the following general formula (IIIa) or (IIIb), or a hydroxycarboxylic acid represented by the following general formula (Ia) or (Ib) A method for improving the optical purity of a derivative. (In the formula, R 1 represents a C 1-8 alkyl group, and R 2 represents an alkali metal or a secondary amine.)医薬、農薬及び工業製品の製造原料として有用な光学活性な２－ヒドロキシカルボン酸の光学純度向上法を提供する。 下記一般式（Ｉａ）又は（Ｉｂ）で表されるヒドロキシカルボン酸を、溶媒存在下、アルカリ金属アルコキシド、及び２級アミンからなる群より選ばれる少なくとも１種の非光学活性な塩基と反応させた後、再結晶し、下記一般式（IIIａ）又は（IIIｂ）で表されるヒドロキシカルボン酸の塩を得る工程を含む、下記一般式（Ｉａ）又は（Ｉｂ）で表されるヒドロキシカルボン酸又はその誘導体の光学純度向上方法。（式中、Ｒ1はＣ1-8アルキル基を示し、Ｒ2はアルカリ金属又は２級アミンを示す。）
