The problem of surface electromagnetic wave excitation of circular waveguide with impedance flange was solved. Initial electromagnetic field set as circular waveguide main mode H₁₁. Using variational principle in single mode approach equations for energetic factor of surface wave excitation and complex factor of reflection from the aperture received. Discovered that factor of surface wave excitation may arrived of maximal and minimal significances for definite correlation between wavelength, waveguide radius and flange impedance. Shown that Inductive impedance gained factor of reflection from the aperture and that difference from ideal conductive cases most outstanding near critical frequency of waveguide.