To provide a green liquor system scale formation inhibitor capable of sufficiently inhibiting scale formation in a green liquor production system, and to provide a method for inhibiting such scale formation using the inhibitor.

The green liquor system scale formation inhibitor includes: itaconic acid and/or a salt thereof as monomer unit; and a polymer with a weight-average molecular weight of 500-150,000. The method for inhibiting the scale formation includes: introducing the inhibitor into the fluid in a green liquor system.