In this work, the scattering of the TM waves by a homogeneously filled slit in a thick impedance screen (see Figure 1) has been analyzed through the Wiener-Hopf method. The contribution of our work lies in the boundary conditions imposed on the thick screen walls. It is supposed that the lateral walls of the slit and horizontal walls of the thick screen have different surface impedances. The results to be obtained by this work will enable us to carry out the effects of the multiple scattering processes arising between slit edges and the influence of the surface impedances and material filling of the thick slit on the scattering phenomenon. In order to validate the present Wiener-Hopf solution experimental verification of the filled slit in a thick impedance screen has been implemented in microwave frequencies with good agreements.