To obtain the subject useful compound applicable to various fields due to excellent surface-active and antimicrobial actions, etc., possessed thereby and its low irritant properties.

A compound represented by formula II [M1 and M2 are each H or a cation; (m) and (n) are each a number of 1-3] is reacted with an acid halide of the formula RCOX (R is a 5-21C alkyl, an alkenyl or a hydroxyalkyl; X is a halogen) and the salt exchange, as necessary, is then carried out to afford an N-long-chain acyliminodibasic acid represented by formula I or its salt. The resultant reactional solution is regulated to pH0-4 with an acid and the prepared reactional solution containing the compound, represented by formula I and kept in a melted state (a liquefied state) is further separated into an aqueous layer and an organic layer. The objective compound contained in the organic layer is separated and obtained therefrom. The resultant compound, as necessary, is neutralized and purified into a high purity. A lower alcohol such as ethanol or isopropyl alcohol is preferably added as a demulsifying agent to the reactional solution in the synthetic reaction.