Provided is a process for the production of a fluorine-containing block copolymer, which suppresses the formation of a homopolymer as a by-product and which, regardless of whether the chain transfer constant of the iodine end is large or small, achieves nearly 100% conversion into a block copolymer. The process is characterized by reacting (A) a fluorine-containing polymer which has an iodine atom or a bromine atom at either or both terminals of the backbone chain and/or at a side-chain terminal with (M) a radical-polymerizable monomer in the presence of (C) a sulfur compound represented by general formula (2): (Y1)nH2-nS2O4 (wherein Y1 is a mono- or di-valent metal ion or an ammonium ion; and n is an integer of 0 to 2).