More specifically, in a preferred embodiment of the compound represented by the general formula (b), the residue Xb forms a naphthalene ring together with the adjacent carbon atoms, and the residue Yb forms a pyridine ring together with the nitrogen atom and the carbon atoms that are adjacent thereto. Thus, the metal-containing azo compound is a complex of (6-substituted-2-pyridylazo)naphthol and a bivalent metal, which is represented by the formula (b1)