  In iron pnictides, we find that the moderate electron-phonon interaction due
to the Fe-ion oscillation can induce the critical d-orbital fluctuations,
without being prohibited by the Coulomb interaction. These fluctuations give
rise to the strong pairing interaction for the s-wave superconducting (SC)
state without sign reversal ($s_{++}$-wave state), which is consistent with
experimentally observed robustness of superconductivity against impurities.
When the magnetic fluctuations due to Coulomb interaction are also strong, the
SC state shows a smooth crossover from the s-wave state with sign reversal
($s_\pm$-wave state) to the $s_{++}$-wave state as impurity concentration
increases.
