PURPOSE: To obtain the titled compound useful as a surfactant capable of exhibiting particularly detergency in hard water in high selectivity and yield, by reacting an α-sulfofatty acid derivative with a specific amount of water in a closed system under heating.

CONSTITUTION: An α-sulfofatty acid derivative of formula I (R1 is 2W20C alkyl; A is H, 1W6C alkyl, alkali metal, alkaline earth metal or ammonium ion; M is alkali metal, alkaline earth metal or ammonium ion) is reacted with water in an amount of 20wt% or more, preferably equal amount W4 times, based on the amount of the α-sulfofatty acid in a closed system, e.g. an autoclave, at ≥250°C, preferably 300W350°C for 1W3hr under heating to afford the aimed substance of formula II.