For the clarification of crystallization conditions to obtain agglomerated crystals of spherical MnCO3 having high purity and high packed density, the reactive crystallization with MnSO4 and (NH4)(2)CO3 solutions in high concentration was carried out using a pilot mixing crystallizer. The crystallization method to obtain spherical and compact agglomerated crystals was determined by applying conditions in which the nucleation rate was made high by the high degree of supersaturation and nuclei were agglomerated in the agglomeration zone in the crystallizer. Furthermore, for the production of agglomerated crystals having packed density larger than 2.6 g/cm(3), the mixing of coarse crystals obtained from the crystallizer bottom with fine crystals in the over flowing solution at the weight ratio of 7 : 3 was found to be suitable.