A slag volume evaluation method for a molten metal surface includes calculating an approximation curve indicating a correspondence between a thickness of slag and a density parameter in advance by measuring thicknesses of a plurality of pieces of the slag which float on a surface of a molten metal in a container and differ from each other in thickness, and calculating a value of the density parameter which is correlated to a density in a pixel region corresponding to the plurality of pieces of the slag in a captured image of a molten metal surface in the container; and calculating a volume of the slag by calculating and integrating the thickness of the slag for each of pixels constituting the captured image obtained by capturing an image of the molten metal surface which is an evaluation target, according to a value of the density parameter of each of the pixels and the approximation curve.