An approximate expression is derived for the firstnterms of exact series expansions for the electromagnetic fields inside a metallic spherical shell due to an impinging plane monochromatic wave. The formulas derived are valid if the ratio (n+1) (n+2) / 2Ri|k2| ≪1, wherek2is the propagation constant in the metallic screen andRiis the radius of the inner sphere. Contrary to some existing approximate formulas, the present ones describe the fields also at the resonance frequencies corresponding to the firstnterms. They fail to represent the fields near or at the resonance frequencies corresponding to the (n+1) th term and higher ones.