dust and or corrosive gas the printed circuit board 16 is shielded from and does not contact with such dust and or corrosive gas the spacing between adjacent ones of the printed conductors on the printed circuit board 16 is small and therefore if electrically conductive dust adheres to the printed circuit board 16 insulation between adjacent conductors is degraded however with the above described arrangement such insulation degradation can be suppressed also since the control devices 18 and the display device 20 do not directly contact corrosive gas if any in the cooling air they are hardly corroded since the wall unit 24 of the receptacle 22 is formed integral with the plate 10 a which is part of the control panel 10 with the printed circuit board 26 which is part of the power supply circuit used also as the lid to close the rear opening of the wall unit the power supply apparatus according to this invention is easy to manufacture furthermore the securing members 28 a and 28 b for securing the printed circuit board 16 in the receptacle 22 are formed integral with the wall unit 24 there is no need to use any screws to fix the printed circuit board 16 part of a power supply apparatus according to a.