  We investigate the chiral properties of quenched domain-wall QCD (DWQCD) at
the lattice spacings $a^{-1} \simeq 1$ and 2 GeV for both plaquette and
renormalization-group (RG) improved gauge actions. In the case of the plaquette
action we find that the quark mass defined through the axial Ward-Takahashi
identity remains non-vanishing in the DWQCD chiral limit that the bare quark
mass $m_f\to 0$ and the length of the fifth dimension $N_s\to\infty$,
indicating that chiral symmetry is not realized with quenched DWQCD up to
$a^{-1} \simeq 2$ GeV. The behavior is much improved for the RG-improved gauge
action: while a non-vanishing quark mass remains in the chiral limit at
$a^{-1}\simeq 1$ GeV, the result at $a^{-1}\simeq 2$ GeV is consistent with an
exponentially vanishing quark mass in the DWQCD chiral limit, indicating the
realization of exact chiral symmetry. An interpretation and implications are
briefly discussed.
