Abstract: A multilayer printed circuit board enables needless electro-magnetic radiation to be suppressed. Interlayer insulation materials are arranged in layer-built constitution between respective layers of a mixed wiring layer of a first signal and/or a power supply wiring, a first ground layer, a second ground layer, and a mixed wiring layer of a second signal and/or a power supply wiring. A through-hole for connecting the ground layers with each other is provided adjacently to a through-hole for connecting the signal and/or the power supply between these layers. According to the constitution, a return circuit current route of the signal and the power supply to the ground layers is secured. As a result, a loop made by the current becomes small, thus needless radiation of electro-magnetic wave is capable of being suppressed.

Abstract: To provide a method of pseudogrounding which is capable of protecting against EMI by pseudogrounding communication lines, etc. without necessity of expensive ferrite cores and increasing the number of steps and cost. A wiring which is connected to an LSI (43) on a package and is connected to an external wiring (47) via a connector (461), radio-frequency connected via a capacitance (451, 452) to ta housing (42) in which protection against EMI is achieved so that the radiated noise energy is not higher than EMI restricted value level. This enables said wiring to be pseudogrounded.