Influence of a symmetry breaking in a two-ridged rectangular waveguide section on its frequency response is numerically investigated. Section is formed by two rectangular inserts arranged near each other in a cross section of a rectangular waveguide. It is found that a symmetry breaking results in generation of additional attenuation pole at the section frequency response. A pole generation mechanism is studied within the concept of the spectral theory of open waveguide resonators. It is interpreted as a result of excitation of a number of natural oscillations of the complex frequencies. Possibilities to pro-vide a filtering function with both a lower-frequency and a higher-frequency attenuation poles are shown.