2-nitro-4-chloro-2'-hydroxy-3'-t-butyl-5'-methylazobenzene 23.2 g was added to a mixture of n-butanol 60 g, water 10.6 g and 97% sodium hydroxide 6.9 g, and the resultant mixture was stirred while raising temperature to 65° C. Thereafter, the mixture was cooled to 50° C., and 9-fluorenone 1.5 g was added to the mixture. The reaction mixture was then heated to 90° C. in one hour, and the mixture was stirred at 90°~96° C. for five hours to effect reaction, thus almost all of the azobenzene having disappeared. Thereafter, water 50 ml was added to the reaction liquor, and the reaction liquor was neutralized to pH 8~9 with 62% sulfuric acid 10 g to precipitate a crystal. The crystal thus obtained was filtered to separate the crystal, and the separated crystal was then fully washed with water and further with a small amount of methanol. The washed crystal was then dried, thus producing 20.6 g of 2-(2-hydroxy-3-t-butyl-5-methylphenyl)-5-chlorobenzotriazole-N-oxide having a melting point of 160° to 163° C. at the yield of 93.0%.
