The pairing mechanism and gap structure in Ba122 pnictides have been hotly discussed for a long time as one of the central issues in Fe-based superconductors. Here, we attack this problem by taking account of the vertex corrections (VCs) for the Coulomb interaction U(U−VCs), which are totally dropped in conventional Migdal-Eliashberg formalism. The U−VC in the charge susceptibility induces strong orbital fluctuations, and the U−VC enlarges the orbital-fluctuation-driven attractive interaction. By analyzing the effective multiorbital Hubbard model for Ba122 pnictides, we find that the orbital fluctuations develop in all four d orbitals (t_2g and z² orbitals) by which the Fermi surfaces (FSs) are composed. For this reason, nearly isotropic gap functions appear on all the hole-type FSs, including the outer hole FS around the Z point composed of the z² orbital. In contrast, nodal gap structure appears on the electron FSs for the wide parameter range. The obtained nodal s-wave state changes to a fully gapped s-wave state without sign reversal (s₊₊-wave state) by introducing a small amount of impurities, accompanied by a small reduction in T_c. The present microscopic theory naturally explains the important characteristics of the gap structure of both hole and electron FSs in Ba122 pnictides without introducing any phenomenological pairing interaction.

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