A molten metal component estimation device estimating a carbon concentration of a molten metal, the molten metal component estimation device comprising: an input device configured to receive measurement information about a refining facility including measurement results regarding an optical characteristic at a furnace throat in the refining facility during a blowing process including exhaust gas information and manipulated variable information; a model database configured to store model expressions and model parameters, regarding a blowing process reaction, including at least a decarburization model expression representing a relation between a carbon concentration in the molten metal and the oxygen efficiency in decarburization during the blowing process and model parameters where an oxygen efficiency in decarburization depends on a carbon concentration in a molten metal in the refining facility; and a model determination unit configured to estimate the carbon concentration in the molten metal based on the measurement results; and determine the model expression and the model parameters to be used by a model calculation unit, based on the estimation result, wherein the model determination unit is further configured to determine the model expression and the model parameters to be used in a calculation by a model calculation unit from among the model expressions and the model parameters stored in the model database based on furnace throat optical characteristic information, calculate an emission intensity change rate; detect a time when the emission intensity change rate shifts from a positive value to a negative value as a time when the carbon concentration in the molten metal reaches the critical carbon concentration; and in a region where the carbon concentration in the molten metal is higher than the critical carbon concentration, select a mass input-output calculation expression based on exhaust gas information and manipulated variable information or a model expression and model parameters from the model database; and after the time when the carbon concentration in the molten metal reaches the critical carbon concentration, the carbon concentration in the molten metal entering a region where the oxygen efficiency in decarburization depends on the carbon concentration in the molten metal, select the decarburization model expression and model parameters thereof from the model database; wherein the model calculation unit is configured to calculate component concentrations of the molten metal including the carbon concentration in the molten metal by using the measurement information, the model expressions and the model parameters. an input device configured to receive measurement information about a refining facility including measurement results regarding an optical characteristic at a furnace throat in the refining facility during a blowing process including exhaust gas information and manipulated variable information; a model database configured to store model expressions and model parameters, regarding a blowing process reaction, including at least a decarburization model expression representing a relation between a carbon concentration in the molten metal and the oxygen efficiency in decarburization during the blowing process and model parameters where an oxygen efficiency in decarburization depends on a carbon concentration in a molten metal in the refining facility; and a model determination unit configured to estimate the carbon concentration in the molten metal based on the measurement results; and determine the model expression and the model parameters to be used by a model calculation unit, based on the estimation result, wherein the model determination unit is further configured to determine the model expression and the model parameters to be used in a calculation by a model calculation unit from among the model expressions and the model parameters stored in the model database based on furnace throat optical characteristic information, calculate an emission intensity change rate; detect a time when the emission intensity change rate shifts from a positive value to a negative value as a time when the carbon concentration in the molten metal reaches the critical carbon concentration; and in a region where the carbon concentration in the molten metal is higher than the critical carbon concentration, select a mass input-output calculation expression based on exhaust gas information and manipulated variable information or a model expression and model parameters from the model database; and after the time when the carbon concentration in the molten metal reaches the critical carbon concentration, the carbon concentration in the molten metal entering a region where the oxygen efficiency in decarburization depends on the carbon concentration in the molten metal, select the decarburization model expression and model parameters thereof from the model database; wherein the model calculation unit is configured to calculate component concentrations of the molten metal including the carbon concentration in the molten metal by using the measurement information, the model expressions and the model parameters.