Fluorine-containing resin composition having a low refractive index

An active energy ray-curable resin composition which gives a cured product having a refractive index of not more than 1.44 comprises (I) a fluorine-containing polymer composed of (A) a fluorine-containing (meth)acrylate, (B) an alpha,beta-ethylenically unsaturated dicarboxylic acid ester and or (C) a mono(meth)acrylate and having a fluorine atom content of at least 30% by weight, (II) a fluorine-containing (meth)acrylate, (III) a mono(meth)acrylate and (IV) a poiy- functional monomer containing at least two (meth)acryloyl groups in the molecule. Optical fibers are produced by coating the above composition on a core material and irradiating active energy rays onto the coating to form a cured cladding of the composition.