Resonant properties of semi-infinite place and rectangular waveguides in the presence of a thin conductive diaphragm placed near the cross-wall are investigated. The diffraction problems of electromagnetic waves by diaphragms are reduced by the method of integral-series identities to the infinite sets of linear algebraic equations. It is found that at some values of the frequency of the excitatory wave there is a sharp resonant increase of the amplitude of standing waves in the area between the diaphragm and the flange.