The problem of H₁₀-wave scattering from the thin wire piece in a rectangular waveguide is solved. The wire penetrating through a small aperture in broad wall can couple the waveguide and free space or another waveguide. The current along the wire is approximated with a number of space harmonics amplitudes of which are found using Galerkin method. The scattering parameters (reflection and transmission coefficients) are found from the excitation theory based on Lorenz lemma. The test calculations are carried out.