PROBLEM TO BE SOLVED: To obtain an additive comprising a specific fluorine-containing polymer for imparting water/oil repellency to a resin, good in initial water/oil repellency, the sustainability of the effect and the compatibility with the resin. SOLUTION: This additive comprises a fluorine-containing polymer obtained by polymerizing (A) a monomer having a polyfluoroalkyl group singly or with (B) a copolymerizable monomer in the presence of (C) an α-methylstyrene dimer, wherein e.g. the component A is a (meth)acrylate having a polyfluoroalkyl group of the formula, etc., and the component B is 2-ethylhexyl (meth)acrylate, etc., in the weight ratio of the component A to the component B of (100 to 0) to (20 to 80), while the component C is preferably added at 0.1-10.0wt.% based on the total weight of the monomers.