  We rigorously investigated the charge-charge correlation function of the
ionic Hubbard model in two dimensions by reflection positivity. We prove the
existence of charge-density waves for large staggered potential $\Delta$ (i.e.,
$\frac{\Delta}{2}+2V>U$) at low temperatures, where $U$ and $V$ are the on-site
and nearest-neighbor Coulomb repulsions, respectively. The results are
consistent with previous numerical simulation results. We argue that the
absence of charge-density waves for $\Delta=0$ and $U$ are large enough (i.e.,
$U>\frac{\Delta}{2}$+2V).
