A shielding method which utilizes a three dimensional structure and is effective to a source of the electromagnetic radiation noise, and a circuit board employing the same are obtained. Further, for the purpose of making the shielding function at an enclosure level unnecessary and realizing the recycling of enclosure materials by using this circuit board, in a circuit board structure having at least a signal layer, a power source layer and a ground layer, a signal line on the signal layer which is sandwiched between the two conductor layers made up of the power source layer and the ground layer, or the power source layers or the ground layers, is enclosed in a three dimensional manner with the conductor layers, thereby to form a single or double electrical closed loop current path or paths. By adopting this structure, there is obtained the effect that the electromagnetic radiation noise radiated from the circuit board is greatly reduced. In addition, as a result, there is obtained the effect that since the shielding processing required for the enclosure becomes unnecessary, the enclosure materials can be recycled.