By the ab initio calculation using the superlattice geometry with neutral interfaces and the repeated-slab geometry with virtual-hydrogen-terminated surfaces, the electronic structures of hexagonal ZnO/GaN(0001) interfaces are investigated. The ZnO/GaN system has type-II band alignment, where both the valence-band top and the conduction-band bottom of ZnO are located below those of GaN. The calculated valence-band offset is around 1.6 eV on average, but it varies from 1.0 to 2.2eV depending on the interface growth treatment. It is shown that the excess charges originating from the heterovalent bondings are strongly localized at the interface.