  Based on the five-orbital model, we study the effect of local impurity in
iron pnictides, and find that the interband impurity scattering is promoted by
the d-orbital degree of freedom. This fact means that the fully-gapped
sign-reversing s-wave state, which is predicted by spin fluctuation theories,
is very fragile against impurities. In the BCS theory, only 1% impurities with
intermediate strength induce huge pair-breaking, resulting in the large in-gap
state and prominent reduction in Tc, contrary to the prediction based on simple
orbital-less models. The present study provides a stringent constraint on the
pairing symmetry and the electronic states in iron pnictides.
