A polymer having a structure represented by general formula: (I) (wherein X, Y, and Z each is hydrogen, fluorine, chlorine, bromine, or iodine, provided that at least one of X, Y, and Z is fluorine; R is linear or branched fluoroalkylene optionally containing oxygen; x and y each represents mol% and x is 1 to 100 mol%; A is -CN, -NCO, -COOR' (R' is hydrogen or alkyl), an acid anhydride group, or an unsaturated hydrocarbon group; and M is a repeating unit derived from a copolymerizable monomer) is treated with a crosslinking agent selected among ammonia, a diamine, and a polyol to crosslink the polymer through pendant functional groups of the polymer. This method of crosslinking can be carried out at room temperature and gives a transparent fluororesin having excellent heat resistance as compared with conventional transparent thermoplastic resins.