Electromagnetic wave penetration into narrow circumferential slots on a conducting circular cylinder is theoretically investigated and rigorous analytic formulations are derived. The Green`s function is used to evaluate electromagnetic wave scattering from the delta electric point source outside of a conducting circular waveguide and theoretical analysis is performed by using the electric and magnetic vector potentials for field representations. The scattered fields are represented in terms of discrete and continuous modes based on the eigenfunction expansion and Fourier transform. The boundary conditions are enforced to obtain a set of simultaneous equations for discrete modal coefficients. Residue calculus is applied to transform the scattered field integral representations into numerically efficient forms. Numerical computations illustrate the behavior of penetrations in terms of the slot geometry, operating frequency, and point source`s position. Our theoretical model for the penetration into multiple circumferential slots is useful for the analysis of EMI and its applications.