We investigate a chiral property of the domain-wall fermion (DWF) system using the four-dimensional hermitian Wilson-Dirac operator H-W. A formula expressing the Ward-Takahashi identity quark mass m(5q) with eigenvalues of this operator is derived, which well explains the N-5 dependence of m(5q) observed in previous numerical simulations. We further discuss the chiral property of DWF in the large volume in terms of the spectra of H-W.