PURPOSE: To obtain the subject compound simply in high yield, by treating a feedstock 2,4-diaminobenzenesulfonic acid with a specific acylating agent, reacting the acylated product with an alkylating agent and hydrolysing the resultant product.

CONSTITUTION: The first step comprises treating a feedstock which is 2,4- diaminobenzenesulfonic acid with an acylating agent derived from a 2-8C alkanoic acid (especially acetic anhydride is preferably used in 1.05-1.15 moles per the feedstock) to give an acyl derivative of formula I (R2 is 2-8C alkanoyl). The second step comprises reacting the product with an alkylating agent of formula II (R1 is 1-4C alkyl) optionally in the presence of an alkali metal halide or an alkali metal sulfate to give a phenylenediamine derivative of formula III. Then, the third step comprises removing the 2-8C alkanoyl group by hydrolysis to give a compound of formula IV.