Calcium carbonate in the form of aragonite crystals suitable for use as a filler for papermaking to give useful properties for bulk, whiteness, opacity, wire abrasion and yield is obtained by taking advantage of the causticization step. Herein disclosed is a process for producing calcium carbonate in the form of aragonite crystals which is useful as a filler for papermaking via the causticization step of the sulfate or soda pulping process, comprising adding an aqueous alkaline solution having a hydroxide ion level of 3 mol/l or less and containing 0.25 mol or less of carbonate ion per 1 mol of quick lime to quick lime and/or slaked lime at a quick lime concentration of 1 60% by weight with stirring to prepare milk of lime, and then adding green liquor to said milk of liquor at a loading rate of 0.002 0.12 g (sodium carbonate)/min/g (quick lime) to perform a causticization reaction at a temperature of 30 105.degree. C.