A high voltage power source device which comprises an electric power source; a low voltage circuit constituted by an inverter circuit including a semiconductor switching element and a capacitor for converting an electric power from the electric power source into a high frequency electric power; a magnetron; a high voltage transformer for converting the high frequency electric power into a high voltage and supplying the high voltage to the magnetron; and a printed circuit board having an electric wiring system formed thereon for connection with and support thereon of the inverter circuit and a high voltage circuit for the voltage boosted by the high voltage transformer. The printed circuit board has separate areas defined therein, and the low and high voltage circuits are formed respectively on the separate areas of the printed circuit board. The printed circuit board includes a patterned grounding conductor formed at the boundary between the sepa­rate areas and also has either a plurality of openings defined therein so as to extend completely through a thickness of the printed circuit in alignment with the patterned grounding conductor, or a plurality of generally elongated electrocon­ductive pieces each having opposite ends connected with the patterned grounding conductor.