In recent years, tooth decay has been found to be caused by the activity of bacteria and foods on the surface of teeth. In order to clarify the cause of tooth decay, contact angles on the tooth surface of oleic acid and linoleic acid, which are main components of lipids in foods, were measured, in addition to that of nicotine. The surface of enamel used was prepared to receive a calcium hydroxyapatite product (ASAHI PRECISION). Each solution was prepared by disolving lipids in artificial saliva preparations of sodium carboxymethyl cellulose (cmc-Na) or hydropropyl cellulose (HPC) dissolved in pure water. The nicotine solution was prepared in a similar manner. The contact angles increased as the concentration of lipids increased. From these findings, the maximum contact angles in cmc-Na and HPC were determined to be 71 degrees and 37 degrees, respectively. In the case of nicotine, the contact angle was measured below 20 degrees over a wide range of concentrations.