An image forming apparatus and a voltage supply method are provided which are capable of fulfilling a stable bias supply to a fittable-and-removable image forming unit and moreover offering improved accessibility to the board. The image forming apparatus includes a lower casing, an image forming unit, a high-voltage board, and a left interconnecting unit. The high-voltage board has a plurality of electric components and output terminals in its upper surface portion above the image forming unit. When a top cover of the lower casing is removed off, the high-voltage board is exposed. The left interconnecting unit is fitted to a side end portion of the high-voltage board and electrically connected to the output terminals of the high-voltage board. The left interconnecting unit supplies a voltage to the image forming unit via a side portion of the high-voltage board.