An image forming apparatus includes at least one image forming section, which includes an image bearing body, a charging unit that charges an area on a surface of the image-bearing body, and a developing unit that supplies a developer material to the charged area to form a developer image on the image bearing body. The developer image is transferred from the image-bearing body onto a transporting unit. A detector detects a density of the developer image on the transporting unit. The controller performs a charging voltage correcting operation in which the controller provides a test charging voltage to the charging unit for forming the developer image, and then a normal charging voltage for normal printing is determined based on the density. The controller also performs a developer-supplying voltage correcting operation in which a developer-supplying voltage supplied to a developer supplying unit is determined based on the density.