A polymer which has a flexible structure in its main chain and thus exhibits a high toughness and can difficultly be deteriorated in its mechanical properties and thermal properties even when sulfonated, a sulfonic acid group-containing polymer obtained by the sulfonation of the polymer, and a proton-conductive membrane having an excellent mechanical strength and durability made of the sulfonic acid group-containing polymer. A novel halogenated aromatic compound represented by the following general formula (1m) is provided: 1 wherein A independently represents an electron-withdrawing group; B independently represents an electron-donating atom or group; X represents a chlorine atom, iodine atom or bromine atom; R1 to R8 may be the same or different and each represent a hydrogen atom, fluorine atom or alkyl group; and n represents an integer of 2 or more.