Azo compound and salt thereof

An azo compound represented by the following formula (1):

• wherein A denotes a phenyl group having 1 to 3 substituents selected from the group consisting of sulfonic acid group, amino group, substituted amino groups, lower alkyl groups, lower alkoxyl groups, hydroxyl group and carboxyl group, or a naphthyl group which has 1 to 3 sulfonic acid groups as substituents and which may have hydroxyl group, B denotes hydrogen atom, sulfonic acid group, a lower alkyl group, a lower alkoxyl group, a halogen atom or nitro group, each of R1 to R4 independently denotes hydrogen atom, a halogen atom, a lower alkyl group, a lower alkoxyl group or acetylamino group, D denotes —NHCO—, —N═N— or —NH—, E denotes hydrogen atom, a lower alkyl group or a phenyl group having 1 to 3 substituents selected from the group consisting of hydroxyl group, amino group, nitro group, sulfonic acid group, carboxyl group, a lower alkyl group and a lower alkoxyl group, n denotes 0 or 1, and m denotes 0 or 1, or a salt thereof, or a copper complex compound of either of them.