  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 $H_W$.
