To obtain the subject compound that is useful as a bulk dye for plastics or as a precursor for production of diisocyanate by allowing naphthalenesulfonic acid or its salt to react with an alkali in the presence of an alkylated urea.

The subject compound of formula I (n is 1-6; X is a 1-6C alkyl; m is 0-6) is prepared by allowing a naphthalenesulfonic acid of formula II (X1 is same as X; SO3H is substitutable to the aromatic OH group) or its salt to react with an alkali in the presence of an alkylated urea derivative. As an alkylated urea derivative, are cited an open-chain urea derivative or a cyclic urea derivative, particularly N,N'-dimethylethylene urea of formula III and it is used in an amount of 0.005-10 pts.wt. per 1 pt.wt. of the compound of formula II. The alkali is preferably an alkali metal hydroxide as KOH and the amount is 1-3 pts.wt. per 1 pt.wt. of the compound of formula II. The reaction is carried out at 150-180°C under the pressure of 1-5 bar.