A process for producing 3-(2-thienylthio)butyric acid comprising converting a compound represented by general formula (III), whereby the formation of by-product 3-(3-thienylthio)butyric acid as a positional isomer of the target compound can be reduced to 0.1 % by mole or below, wherein R represents a linear or branched C1-C4 alkyl. The obtained 3-(2-thienylthio)butyric acid is useful as an intermediate of medicinal compounds.
