The band calculations of noble metal halides are studied to reveal the high ionic conductivity of Ag+ and Cu+ in noble metal halides using the linear combination of atomic orbitals (LCAO) theory. It is found that the d states of Ag+ are much more weakly coupled with the p states of halogen ions, while those of Cu+ are much more strongly coupled with the p bands. The strength of p-d hybridization is discussed in relation to the activation energy for the ionic conduction. It is shown that the high ionic conductivity of AgX (X = halogen) primarily stems from the combination of the deformability of the d shell and the weakness of p-d hybridization.