Obtaining a crosslinked rubber with high flexibility at low temperature includes crosslinking a fluororubber composition. The fluororubber composition includes a fluororubber and a compound of formula (X—)x(Z—)zY. In the formula, X is U—(CF2)aO(CF2CF2O)b—; Z is RFO(CF2CF2O)c—; Y is a (x+z) valent perfluoro saturated hydrocarbon group, optionally interrupted by an etheric oxygen atom; x is an integer of at least 3; z is an integer of at least 0; U is a monovalent group with an unsaturated hydrocarbon, a bromine atom, an iodine atom, or a combination thereof; RF is a C1-20 linear perfluoroalkyl group, optionally interrupted by an etheric oxygen atom; a is an integer of from 0 to 20; b is an integer of from 1 to 200; and c is an integer of from 3 to 200.