Heat transfer in continuous casting mold is important to decide the surface quality of the cast slab of middle carbon steel. Many researchers have recently studied the mechanism of reducing the heat transfer between the mold and the solidified shell, and some of them have pointed out that the interfacial thermal resistance between the mold and surface of solidified mold flux has caused decreasing heat transfer. In the present study, the surface roughness of solidified mold fluxes used for low carbon and middle carbon steel casting, Na2O-CaO-SiO2 and Li2O-CaO-SiO2 slag systems, was measured by a confocal scanning laser microscope combined with an infrared image furnace. It was found that the surface roughness was in the range of approximately 10-30 μm when the crystalline phase precipitated. Furthermore, the faster the cooling rate, the smoother the surface roughness of the solidified mold flux became, in the contrast, the surface roughness of the slag, of which critical cooling rate is faster, increased. The surface roughness of the mold flux for middle carbon steel casting became rougher than that for low carbon steel casting. As a result, the surface roughness was related to normalized cooling rate, which is the ratio of actual experimental cooling rate to critical cooling rate. The experimental data of surface roughness were fairly in agreement with calculated values on the assumption of one dimensional heat transfer in the continuous casting mold.