A process for preparing β-mercaptocarboxylic acid comprising: Step a for reacting a compound represented by a formula: X 1 2 S (X 1 represents hydrogen, Na or K) or a compound represented by a formula: X 2 SH (X 2 represents Na or K), alkali hydroxide represented by a formula: X 3 OH (X 3 represents Na or K), and unsaturated carboxylic acid represented by the following General Formula to obtain a reaction solution including a compound represented by the following General Formula and a compound represented by the following General Formula; Step b for neutralizing the reaction solution obtained in Step a with an acid to give β-mercaptocarboxylic acid in an aqueous layer, and then extracting the β-mercaptocarboxylic acid from the aqueous layer with organic solvent to obtain a solution including β-mercaptocarboxylic acid represented by the following General Formula and a compound represented by the following General Formula; Step c for distillation-refining the extracted solution obtained in Step b to obtain the β-mercaptocarboxylic acid represented by General Formula, the distillation ending at a state in which β-mercaptocarboxylic acid is contained in the range of 5 wt% to 50 wt% in a distillation residue including the compound represented by General Formula; and Step d for returning the distillation residue in Step c to Step a. wherein, in Formula, each of R 1 and R 2 represents hydrogen or a C1 to C4 alkyl group, and may be the same as or different from each other, wherein, in Formula, R 1 and R 2 have the same definition as in Formula, X 4 represents Na or K, X 4 of which two are present may be the same as or different from each other, wherein, in Formula, R 1 and R 2 have the same definition as in Formula, X 4 represents Na or K, R 1, R 2 or X 4 of which two are present respectively may be the same as or different from each other, wherein, in Formula, R 1 and R 2 have the same definition as in Formula, and wherein, in Formula, R 1 and R 2 have the same definition as in Formula, R 1 and R 2 of which two are present respectively may be the same as or different from each other. Step a for reacting a compound represented by a formula: X 1 2 S (X 1 represents hydrogen, Na or K) or a compound represented by a formula: X 2 SH (X 2 represents Na or K), alkali hydroxide represented by a formula: X 3 OH (X 3 represents Na or K), and unsaturated carboxylic acid represented by the following General Formula to obtain a reaction solution including a compound represented by the following General Formula and a compound represented by the following General Formula; Step b for neutralizing the reaction solution obtained in Step a with an acid to give β-mercaptocarboxylic acid in an aqueous layer, and then extracting the β-mercaptocarboxylic acid from the aqueous layer with organic solvent to obtain a solution including β-mercaptocarboxylic acid represented by the following General Formula and a compound represented by the following General Formula; Step c for distillation-refining the extracted solution obtained in Step b to obtain the β-mercaptocarboxylic acid represented by General Formula, the distillation ending at a state in which β-mercaptocarboxylic acid is contained in the range of 5 wt% to 50 wt% in a distillation residue including the compound represented by General Formula; and Step d for returning the distillation residue in Step c to Step a. wherein, in Formula, each of R 1 and R 2 represents hydrogen or a C1 to C4 alkyl group, and may be the same as or different from each other, wherein, in Formula, R 1 and R 2 have the same definition as in Formula, X 4 represents Na or K, X 4 of which two are present may be the same as or different from each other, wherein, in Formula, R 1 and R 2 have the same definition as in Formula, X 4 represents Na or K, R 1, R 2 or X 4 of which two are present respectively may be the same as or different from each other, wherein, in Formula, R 1 and R 2 have the same definition as in Formula, and wherein, in Formula, R 1 and R 2 have the same definition as in Formula, R 1 and R 2 of which two are present respectively may be the same as or different from each other.