To obtain the aqueous solution of a diazonium salt in a high yield by a simple method comprising the diazotization of an aromatic amine with a diazotization agent, wherein the aromatic amine is obtained by reacting the aqueous solution of the water-soluble salt of an aromatic nitro compound with hydrogen in the presence of a specific catalyst.

This method for producing an aqueous solution of a diazonium salt of formula II (at least a part of amino groups is a protonated form; Y- is F-, formate or the like) comprises reacting (A) the aqueous solution of the water-soluble salt of an aromatic nitro compound of formula I [R1 to R4 are each H, a 1 to 16C alkyl or the like; A, A' and A" are each O, S, CO or the like, B, B' and B" are each a (substituted) 1 to 14C alkylene, a (hetero)arylene or the like; (k), (l) and (m) are each 0 to 3; (n), (o) and (p) are each 0 to 1; (x) is 1 to 3; (y) and (z) are each 1 to 4, provided that (x)+(y)+(z) is 3 to 5] with (B) hydrogen in the presence of (C) a catalyst for catalyzing the reduction of the aromatic nitro group into the amino group and then diazotizing (D) the obtained aromatic amine with (E) a diazotization agent.