To suppress adverse reaction and simply obtain the subject compound useful as a synthetic intermediate for dyes, etc., in photochemical field in high purity by hydrolyzing an N-substituted-1,2,4-triazole derivative halide with sulfonic acids.

(A) A compound of formula I [R1 is a (substituted)aliphatic group or a (substituted)aryl; R2 is an electron-attracting group: R3 is a group substitutable for nitrogen atom of triazole ring (e.g. acetyl or benzoyl); X is a halogen such as Br] is hydrolyzed with (B) sulfonic acids (e.g. substituted with an aliphatic group or aryl) to provide the objective compound of formula II. The reaction is preferably carried out by using the component B in 1.0-10.0 equivalent amount based on the component A in a nonpolar solvent at 0-120°C.