Electromagnetic plane wave scattering by a loaded trough on a ground plane has been analyzed by Kobayashi and Nomura's method. The field in each region is expressed first in terms of appropriate eigen functions, whose excitation coefficients are determined by the continuity condition across the aperture of the trough. Simple far field expression which is suitable for numerical calculation for small aperture cases has been derived. Scattering far field patterns and radar cross section are calculated and compared with those obtained by other methods. Good agreements have been observed for all incident angles.