Recently two phenomena concerning the instability of Prototypes of Kilogram have been disclosed by gravimetric technique ; one is the change in the mass related to the ambient relative humidity and the other is the noticeable mass increase lasting about a year after cleaning. To elucidate these phenomena atomically, surface analyses such as FT-IR and XPS were made. For the former phenomenon, even under a vacuum of 10^-2 Pa, the order of the -OH peak values for pure Al. Pt and Au was in agreement with that of weight gain of these metals. For the latter, it was clarified that about 50% organic carbon was removed during supersonic cleaning in solvents, and that the surfaces are covered once again in 6 hours. The mass changes by cleaning were estimated about 10 μg in the case of the Prototype.