The problem of the dominant ТЕ₁₀ mode diffraction by an iris with a thin stepped slot placed in a rectangular waveguide is considered. Placing such an iris into a rectangular waveguide cross-section affects on the appearance of the TE₁₀ mode total transmission resonance in the single mode frequency band. Influence of the geometrical dimensions on the resonance frequency and Q-factor was studied. An example of a three-section bandpass filter basing on such the irises as a resonant sections is presented as well.