A water-soluble thickener which is highly effective even in thickening strongly acidic aqueous solutions and has excellent stability in such solutions. It comprises a water-soluble copolymer having a weight-average molecular weight of 6,000,000 or higher obtainable by polymerizing a monomer mixture which comprises 2-acrylamido-2-methylpropanesulfonic acid and/or a salt thereof and acrylic acid and/or a salt thereof as essential components and optionally one or more other copolymerizable monomer components including the compound represented by the following Formula (1) and/or a salt thereof, wherein the 2-acrylamido-2-methylpropanesulfonic acid and/or a salt thereof represents 20 mol % or more of all the monomers. ##STR00001## (wherein n is an integer of 1 to 12).