Attenuated total reflection (ATR) properties and scattered light properties were measured for Ag thin films and arachidic acid (C20) Langmuir-Blodgett (LB) ultrathin films on the Ag thin films to obtain the information about their complex dielectric constants and surface roughness utilizing an excited surface plasmon polariton. The complex dielectric constants for the Ag thin films and the C20 LB films were obtained by fitting the calculated ATR curves to the experimental ones. The surface roughnesses of these films were estimated by the angular distribution of the scattered light assuming the Gaussian function as an autocorrelation function and a linear superposition of roughness spectra. The angular spectra strongly depended on the roughness parameters: the transverse correlation length σ and the surface corrugation depth δ. The experimental angular distributions were explained by some pairs of σ and δ. It was suggested that the surface roughness of the C20 LB films changed with the number of monolayers since the angular spectra varied with the number of the C20 LB monolayers on the Ag films. It is thought that the measurement of the scattered light is useful to evaluate surface roughnesses of LB ultrathin films.