This paper presents a field penetration characteristic, into a metallic wall with a narrow slot, due to a nearby dipole source. Coupled integral equations are derived and solved by applying Galerkin's method of moments (MoM) for calculating the penetrating electric field. It is shown that the 26dB attenuation level field penetration into the narrow slot occurs at the dipole source position of much more than about 0.6λ separation distance along the lateral direction from the slot center. It is also found that the 30dB attenuation levels field penetration appear at the observation positions of much more than about 0.5λ along the direction of the slot length and about 1.07λ separation distance along the direction of the slot width from the slot center.