To control the phosphorus concentration in molten pig iron after a preliminary treatment of molten pig iron to achieve a target value with a high accuracy without much effort.

A basicity calculation part 31 calculates, as a degree of similarity, a distance between operation conditions of an estimated charge and operation conditions of past charges stored in an operation record database 2, uses the operation conditions and slag basicity of the past charges and the calculated degree of similarity to prepare a regression formula for calculating the slag basicity in the preliminary treatment of molten pig iron, and uses the formulated regression formula to calculate the slag basicity when performing the preliminary treatment of molten pig iron under the operation conditions of the estimated charge. Then, a lime charge calculation part 32 calculates the amount of lime required for performing the preliminary treatment of molten pig iron of the estimated charge, based on the slag basicity calculated by the basicity calculation part 31. Then, a lime charge control part 33 controls the amount of lime charged into the molten pig iron, based on the amount of lime calculated by the lime charge calculation part 32.