A fluorosulfonyl group-containing compound having a high polymerization reactivity, a process for its production, a sulfonyl group-containing polymerizable monomer led from the sulfonyl group-containing compound, and a polymer obtainable by polymerizing the sulfonyl group-containing polymerizable monomer, are provided.A compound (3) is fluorinated to form a compound (4), and then, the compound (4) is subjected to a decomposition reaction to produce a compound (5). A preferred compound (5-1) of the compound (5) is thermally decomposed to produce a compound (7-1) having a high polymerization reactivity. ##STR00001## wherein R.sup.A is a bivalent organic group such as a fluoroalkylene group, R.sup.AF is a group having R.sup.A fluorinated, or the same group as R.sup.A, each of R.sup.B to R.sup.D which are independent of one another, is a hydrogen atom, etc., each of R.sup.BF to R.sup.DF is a fluorine atom, etc., R.sup.E is a monovalent organic group, R.sup.EF is a group having R.sup.E fluorinated, or the same group as R.sup.E, E is a bivalent connecting group, E.sup.F is the same group as E, or a group having E fluorinated, E.sup.F1 is a group formed by scission of E.sup.F, each of X.sup.1 to X.sup.3 is a hydrogen atom, etc., and each of X.sup.1F to X.sup.3F is a fluorine atom, etc.