To provide a technique to improve a whiteness degree of causticized light calcium carbonate as a byproduct in a causticizing process for regenerating a cooking white liquor in a pulp production process by a sulfate method or a soda method.

In a causticizing process of a pulp production process by a sulfate method or a soda method, a green liquor is clarified to a high level by adding an anionic polymer flocculant having a molecular weight of 13×106 to 20×106 calculated from the intrinsic viscosity and having a charge amount of anionic monomers of 8 to 15 mol%, to the green liquor. Thereby, the whiteness degree of causticized light calcium carbonate produced by a causticizing reaction of the clarified green liquor with calcium oxide or calcium hydroxide can be stably increased.