An image forming apparatus forming a preferable image without white dots caused by excessive charging when a halftone image is formed by providing a potential difference between a surface potential of a photosensitive drum immediately before charging by a charging device, and that of the drum after the charging in an image forming area of the drum. At least one of a charging bias and a transfer bias is changed between a first area and a second area. The first area is an area on the drum that passes a transfer portion when a recording medium is not present between the roller and the drum, and on that a latent image is to be formed. The second area is an area on the drum that passes the transfer portion when the medium is present between the roller and the drum, and on that a latent image is to be formed.