The radiation behavior of a parallel-plate-fed slit in a thick conducting screen is examined. The Fourier transform and the mode-matching technique are used to obtain simultaneous equations for the transmitted field inside the thick conducting screen. The simultaneous equations are solved to represent the transmitted and scattered fields in simple series forms. The numerical computation is performed to illustrate the behavior of the radiation from the parallel-plate-fed slit. A substantial reduction in the reflection coefficient is possible by choosing a thickness of the conducting screen.