To provide a coating composition useful to form an antireflection film having a refractive index lower than 1.4, excellent in water solubility and long-term stability in the form of a water solution. A coating composition containing a fluoropolymer having ionic functional groups (such as a fluoropolymer having polymer units of --CF.sub.2CF[O(CF.sub.2).sub.3COOH]--), an acid and an aqueous medium.