A long-chain n-alkane, tetratetracontane (n-C44H90), adsorbed on Ag(111) was studied by infrared reflection absorption spectroscopy. The first layer is physisorbed on the surface and assumes a "flat-on" structure with the molecular plane formed by the carbon atoms parallel to the surface. This structure gives CW stretching vibration bands at 2907 and 2814 cm(-1), which are attributed mainly to the CH bonds in the upper side of the molecular plane and those in the vicinity of the Ag(lll) surface, respectively. These frequencies are lower than those for the normal stretching modes of a methylene group, and this difference is ascribed to the interaction between the CH bonds on the metal side and the Ag surface. This "softening" of the CW bond in contact with the surface was explained by the interaction between the dipole and the image dipole induced in the metal substrate, and this frequency shift of the CH stretching vibration was simulated by electrodynamic calculations.