The invention provides a process for producing an optically active butyric acid compound and a production intermediate therefor at high yield and high purity. The present invention provides a process for producing Compound (6), including reacting Compound (1) with optically active 2-trifluoromethanesulfonyloxybutyrolactone (2a) in the presence of a base or reacting optically active 2-hydroxybutyrolactone (2b) under Mitsunobu reaction conditions, to thereby form Compound (3); reacting Compound (3) with an alcohol and a halogenating agent, to thereby form Compound (4); dehalogenating Compound (4), to thereby form Compound (5); and de-esterifying Compound (5). ##STR00001##