To prevent invasion of a radio signal into an electromagnetic shielding space through e.g. a ventilation port or leakage of the radio signal from the electromagnetic shielding space into the exterior.

In an electromagnetic shield 1, a honeycomb mesh of a plurality of cylinders 3 made of conductive material and arranged in parallel is disposed so as to face a ventilation port of an air conditioning duct or ventilating duct, thus establishing a shield zone of a waveguide mode and causing reactance attenuation. An electromagnetically shielding function depends not only on an aperture dimension of an ring-shaped hole 2 of the cylinders 3 but also on the length of the cylinders. Thus when the aperture and length dimensions of the cylinders 3 are adjusted a desired electromagnetic shielding function can be obtained. Accordingly it can be avoided that electromagnetic waves invade from the exterior into the interior via an air-conditioning or ventilating duct or leak from the interior into the exterior, without sacrificing a ventilating ability due to an air resistance in comparison with a conventional metallic net.