  Energy versus magnetic-field diagram of the spin-$1$ Haldane system with an
impurity bond is studied in terms of spin-1/2 degree of freedom at the sites
neighboring the impurity bond by means of analytical method. We examine the
equivalence between the realistic Hamiltonian and the phenomenological
Hamiltonian which is composed two spin-1/2 spins representing the spin-1/2
degree of freedom. It is proved that when the strength of the impurity bond is
sufficiently weak, the two Hamiltonians are equivalent to each other, as far as
the energies of the low-lying states are concerned. We determine the
correspondence between the interaction constants in the phenomenological
Hamiltonian and those in the realistic Hamiltonian.
