We report the optical (6meV–4eV) properties of a boride superconductor ZrB12 (Tc=6K) in the normal state from 20to300K measured on high-quality single crystals by a combination of reflectivity and ellipsometry. The Drude plasma frequency and interband optical conductivity calculated by the self-consistent, full-potential linear muffin-tin orbital method agree well with experimental data. The Eliashberg function α2trF(ω) extracted from optical spectra features two peaks at about 25 and 80meV, in agreement with specific heat data. The total coupling constant is λtr=1.0±0.35. The low-energy peak presumably corresponds to the displacement mode of Zr inside B24 cages, while the second one involves largely boron atoms. In addition to the usual narrowing of the Drude peak with cooling down, we observed an unexpected removal of about 10% of the Drude spectral weight, which was partially transferred to the region of the lowest-energy interband transition (∼1eV). This effect may be caused by the delocalization of the metal ion from the center of the B24 cluster.