An ultralow refractive index is very advantageous when one designs antireflective coatings. We successfully obtained high-quality {\mathrm{MgF}}_2 thin films with ultralow refractive indices from autoclaved sols prepared from magnesium acetate and hydrofluoric acid. The {\mathrm{MgF}}_2 films consist of nanosized particles, and they have high laser-exposure durability at 193\mathrm{nm} . The reflectance of the antireflective coating with five layers, of which the top layer is formed by our method, is lower than 0.6% in the incident angle range of 0\mathrm{°–}60° at 193\mathrm{nm} .