Scattered light properties from Ag thin films and arachidic acid (C20) Langmuir-Blodgett (LB) films on the Ag thin films were investigated to evaluate the surface roughnesses of these films utilizing the surface plasmon polariton (SPP) excited in the attenuated total reflection (ATR) configuration. The surface roughness of the films was estimated from the angular distribution of the scattered lights. This result was qualitatively corresponding to the evaluation by the atomic force microscope (AFM) measurements.