When supplying oxygen source to molten pig iron inside a converter-type refining furnace and performing desiliconization, dephosphorization, and decarburization refining, one or more of slag removal flow shape, slag removal flow velocity, and slag surface shape while discharging slag through a throat is measured to estimate one or both of a slag removal amount and physical properties of removed slag. When sequentially performing one or both of desiliconization and dephosphorization, an intermediate step of discharging part or all of generated slag through the throat, and the remaining other refining step, in the intermediate step, the method measures one or two of slag removal flow shape, slag removal flow velocity, and slag surface shape, estimates one or both of amount and physical properties of slag removed, estimates remaining slag amount, or remaining slag amount and composition, and determines an auxiliary raw material amount to be fed in the other refining step.