PURPOSE: To dramatically reduce the necessary dispersant amount and to produce a desired product in a good yield with a high purity by carrying out an azo coupling reaction by using a specific dispersant.

CONSTITUTION: A substituted 2-nitro-2'-hydroxy-azobenzene represented by formula III is obtained by reacting a diazonium salt of a 2-nitroaniline represented by formula I (wherein R3 is H or a halogen) in an acid aqueous solution and a phenol represented by formula II (wherein R1 and R2 are each a 1-7C alkyl) dispersed in water by using an 8-20C alkane sulfonic acid and/or the corresponding alkane sulfonate and a polyvinyl alcohol of 10,000 to 250,000 molecular weight, in particular, 30,000 to 70,000 molecular weight at 20 to 60°C. Based on the theoretical amount of a compound represented by formula III, 0.5 to 10 wt.% of the alkane sulfonic acid or the alkane sulfonate and 0.1 to 5 wt.% of the polyvinyl alcohol are used. By reducing and cyclizing the compound represented by formula III, a benzotriazole represented by formula IV, useful as a light stabilizer, can be obtained.