To obtain the subject compound capable of being melted and secondarily processed, by carrying out copolymerization in an aqueous medium in the presence of a water-soluble initiator, a dispersant and a chain transfer agent.

In preparing a partially crystallizable copolymer containing at least 0.3% total unstable part by copolymerizing (D) tetrafluoroethylene with hexafluoropropylene in an aqueous medium in the presence of (A) a water- soluble initiator, (B) a dispersant and optionally (C) a chain transfer agent, the amount of the component A is reduced, a part not more than a half of the copolymer is made to start by the component A, the amount of the component C is increased, the component A is supplemented with the component C in starting of the copolymer, the amount of the component E is adjusted to an amount corresponding to 2.0-5.0 HFPI, (F) the amount of a fluorinated vinyl ether is made 0.2-0.4wt.% and the copolymerization is carried out to give the objective copolymer containing ≤0.2% total unstable part in a polymerized state.