To provide a scale formation inhibitor that can be produced by an inexpensive and simple production system and sufficiently inhibits scale formation in a green liquor production system.

The scale formation inhibitor comprises a copolymer D-H composed of a monomer unit consisting of maleic acid and acrylic acid and having a weight-average molecular weight of ≥3,000. The scale formation inhibitor containing the copolymer D-H is introduced into a fluid in a green liquor production system containing a calcium ion so that precipitation of calcium salt is controlled and scale formation is sufficiently controlled. Preferably the scale formation inhibitor is introduced into a fluid so that the concentration of the copolymer D-H to the fluid is in a range of 0.1-1,000 mg/L.

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