A method and system for predicting slopping in a converter occurring during decarburization refining in the converter in which molten steel is produced from a molten pig iron by blowing oxidizing gas to the molten pig iron in the converter from a top blowing lance, or optionally further blowing oxidizing gas or inert gas from a bottom blowing tuyere to perform the decarburization refining of the molten pig iron. The method includes measuring an emission spectrum of a throat combustion flame blowing out from a throat of the converter, calculating emission intensity of the measured emission spectrum at a wavelength in a range of 580 to 620 nm, and predicting the occurrence of the slopping based on a time-series change of the calculated emission intensity.