The solid-solid interface between NaCl substrate and KCl deposit crystals of a NaCl-KCl combined crystal which was created in the NaCl-KCl-H₂O ternary system was observed. It was assumed that the NaCl-KCl interface had different physical properties from pure NaCl or pure KCl crystal. In order to discuss the formation process and the physical property of the NaCl-KCl interface experimentally, an atomic force microscope (AFM) was applied to observation and analysis. After cleaving the combined crystal, the influence of moisture in the air to the solid-solid interface between NaCl and KCl was examined by controlling humidity. The force-vs.-distance curve of AFM was measured as the line analysis on the cleaved crystal. As the result, it became clear that the NaCl-KCl interface was easily damaged by the moisture in the air. [DOI: 10.1380/ejssnt.2007.113]