  To understand the impurity effect on Tc in FeAs superconductors, we analyze a
simple two-band BCS model with repulsive interband interaction. The realized
fully-gapped superconducting state with sign reversal, which is predicted by
spin fluctuation theories in this compound, is suppressed by impurities due to
the interband hopping of Cooper pairs, if the interband impurity scattering
$I'$ is equal to the intraband one $I$. When $|I'/I| < 1$, in highly contrast,
Tc is almost unchanged by strong impurity scattering since the interband
scattering is almost prohibited by the multiple scattering effect. Since
$|I'/I| \sim 0.5$ is expected, the robustness of superconductivity against
impurities in FeAs superconductors is naturally understood in term of the sign
reversing fully-gapped state.
