A simultaneous estimation of thermal emissivity and skin temperature of a sea surface was conducted for the data set of Mutsu Bay Sea Surface Validation Experiment in 1997 (MUBEX '97). The proposed method uses a thermal image of a sea surface including a floating object such as a spherical float. By combining this image with data of sky radiation temperature measured by another apparatus, we retrieve both skin sea surface temperature and sea surface emissivity from a single thermal image. Since the method requires no knowledge of thermal radiative properties of the actual sea surface, it can be used even for a contaminated sea surface, e.g., oil slicks or slicks produced by biological wastes. We demonstrate an application of this method to the MUBEX data, and consider some error factors in this measurement.