If the polymerization medium is a hydrophilic solvent such as alcohol and ketone or a mixture of water and a hydrophilic solvent, it is impossible to produce the acid-type polymaleic acid (homopolymer) having the acrylic acid structure in the main chain, or it is impossible to produce the acid-type maleic acid copolymer in which maleic acid (A) and water-soluble monomer (B) are evenly copolymerize