AbstractRecently, many conducting plastic shielding materials have been retailed in connection with environmental electromagnetic problems. This paper proposes a method of measuring the shielding effectiveness using a spherical cell as a means of evaluating shielding effectiveness. The shielding effectiveness against electric and magnetic fields can be evaluated by the proposed method. The measured values provide quantitative information for practical shielding because the cell forms a practical shield housing of circuits or parts. It is easy to handle theoretically since we use a spherical shape. Favorable agreement is obtained between the results of theoretical analysis and values of experimental measurements. Gradual decreases of the shielding effectiveness occurred in the high‐frequency region against the electric fields and in the low‐frequency region against the magnetic fields for a material with low electrical conductivity. Sudden large decreases of the shielding effectiveness against the electric and the magnetic fields due to resonant effects were observed when the diameter of the sphere becomes close to the wavelength based on numerical calculations. It was found that in designing shielding housings, careful attention must be given to their dimensi