The precise solubility data of a component in high-temperature melts are essential for the design and optimization of various processes including pyrometallurgical and solution growth processes. In this study, we developed an in situ method for measuring the solubility of a component in molten metals and alloys and investigated C solubility in molten Si using a system comprising Si and SiC substrate. The three-dimensional interfacial shape between the SiC substrate and molten Si droplet was analyzed from images captured by interferometry utilizing the internal interference in the SiC substrate. From the changes in interfacial shape, the temperature dependence of C solubility in molten Si up to 1873 K was successfully evaluated in the single experiment. The obtained C solubilities were in accordance with the smallest solubilities ever reported, suggesting a reliable evaluation because overestimations sometimes happen in the conventional point-by-point measurements of quenching samples. In addition, the excess partial molar Gibbs energy of C in molten Si was evaluated thermodynamically. The proposed method enables the in situ and crucible-free quantitative analysis of the solubility of a component in molten metals and alloys in the order of a few tens to hundreds of ppm for various material systems at high temperatures.