Disclosed is a water and oil resistant agent comprising a fluorine-containing copolymer obtained by copolymerizing a (meth)acrylate monomer having a polyfluoroalkyl group having 1-6 carbon atoms, hydrophilic monomer, and monomer having an anion-donating group, which agent affords a superior water and oil resistance to a paper. The invention also discloses a process for treating a paper by the water and oil resistant agent and a treated paper thereby.