To provide a calcium oxalate scale inhibitor capable of efficiently inhibiting calcium oxalate scale in water system of a paper pulp production process, particularly in water system in which pH is 4-8 and to provide a method for inhibiting calcium oxalate scale.

The calcium oxalate scale inhibitor in water system of the paper pulp production process comprises a copolymer, containing maleic acid, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid as monomer components, in which the molar ratio of maleic acid to acrylic acid is (10:2) to (10:15) and the content of 2-acrylamide-2-methylpropanesulfonic acid is 3-20 mol% based on total monomer content and/or its water-soluble salt. The method for inhibiting calcium oxalate scale comprises adding the copolymer to the water system.