To obtain an anionic surfactant excellent in detergency, biodegradability and solubility at low temperatures, and applicable to detergents for clothing, by mixing an α-sulfofatty acid alkyl ester, a fatty acid (salt) and a specified amount of an alkali neutralizing agent together to effect neutralization.

This anionic surfactant is obtained by mixing together (A) an α-sulfofatty acid alkyl eater such as a compound of the formula (R1 is a 8-22C alkyl; R2 is a 1-3C alkyl), (B) a fatty acid (salt) such as a compound of the formula RCOOM [R is a 9-17C alkyl or alkenyl; M is H, a monovalent- equivalent alkali (alkaline earth) metal or (substituted) ammonium] and (C) such an amount of an alkali neutralizing agent (e.g. powdery one) as to be at least equivalent for neutralizing both the components A and B to effect neutralization and contain the fatty acid salt ≥0.2 in the weight ratio of the unsaturated fatty acid group to saturated fatty acid group in the weight ratio at 0.1-1 based on the salt of the component A.