Poly-azo compound selected from the group consisting compounds of the formula (I) and compounds of the formula (II), ##STR00001## (wherein, Me represents copper, nickel, zinc or iron, A represents a naphthyl group having 1 to 3 sulfonic groups and further optionally having an alkyl group or alkoxy group, R.sup.1 and R.sup.2 represent each independently a hydrogen atom, alkyl group or alkoxy group, and R.sup.3 represents an amino group or hydroxyl group.).