In this paper, an electromagnetic plane wave diffraction by finite number of loaded thick slits on infinitely long perfectly electric conductor (PEC) screen is analyzed. Here we formulate the problem by utilizing the Kobayashi Potential (KP) method, which is a kind of eigenfunction expansion method in terns of Weber-Schafheitlin discontinuous integrals. The multiple scattering contributions between the slits are analytically included in the formulation. The solution derived here may provide us with precise numerical result, so it may be considered as a reference solution to other numerical and approximate analyses.