Provided are a sulfonated polymer capable of high hot water resistance even if it has an increased amount of the sulfonic groups introduced therein, and a solid polymer electrolyte containing the sulfonated polymer that has high proton conductivity and excellent generating performance.The sulfonated polymer has repeating units represented by the formula (1'): ##STR00001## wherein B's are each independently an oxygen or a sulfur atom, R.sup.1 to R.sup.3 may be the same or different and are selected from a hydrogen atom, a fluorine atom, a nitrile group and an alkyl group, n is an integer of 2 or greater, and Q is a structure represented by the formula (q): ##STR00002## wherein A is independently a divalent atom or organic group or a direct bond, and R.sup.4 to R.sup.11 may be the same or different and are selected from a hydrogen atom, a fluorine atom, an alkyl group and an aromatic group.