to 8 carbon atoms, aryl groups each having 6 to 20 carbon atoms or ???SO3X, wherein X is a hydrogen atom, a metal atom, an ammonium group or an amino group, and at least one of R1 to R6is ???SO3X; unsaturated (meth)allyl ether sulfonic acids represented by the following general formula (II) and salts thereof (for example, 3-allyloxy-2-hydroxypropanesulfonic acid and salts thereof, and 3-methallylo