To effectively inhibit the scale precipitation occurring in the alkali recovery process by admixing a water-soluble calcium salt to the water in the alkali recovery process in a paper pulp factory.

A water-soluble calcium salt, for example, calcium chloride is added to the water in the alkali recovery process in an amount of 10-5,000 mg calculated as calcium ion per 1 liter of the water in the process whereby the precipitation of calcium scale of calcium carbonate, calcium carbonate including sodium is previously prevented in the digester, the evaporator, the dissolver tank, the green liquor line, the slaker and the like.