Image forming apparatus having a photosensitive drum 1, a charger 2 for uniformly charging the photosensitive drum 1, an exposing means 3 for forming an electro-static latent image on the photosensitive drum 1, an exposing unit 4 for forming a visual image by developing the electro-static latent image, a transfer roller for transferring the visual image to a printing medium 10, which comprises a transfer voltage supply circuit that supplies the transfer roller 5 one or more times with voltage in one polar direction and successively in the other polar direction during an interval after starting of operation of the image forming apparatus and before the time when the printing medium 10 is transported to the transfer roller 5. By controlling in such a way, an image forming apparatus is provided in which memory effect does not arise, the transfer roller 5 being prevented from having accumulated toner attached thereto, the toner attached to the transfer roller 5 being cleaned, and so the reverse side surface of the printing medium 10 is not stained.