PURPOSE: To obtain an image always free from fogs as compared with latent images with respect to a method for developing the latent images, by using a one-component magnetic developer by applying AC bias upon a developing sleeve in the stage of nondeveloping and applying DC bias upon the sleeve coated with the developer.

CONSTITUTION: A one-component magnetic developer 5 of high insulation characteristic charged negative is charged into a developing hopper 7. While film thickness is regulated with a doctor blade 4, the developer 5 is coatd on a coating sleeve 1. The coating sleeve is rotated as an arrow to stick the toner on the latent images on a photosensitive drum 8 via a developing sleeve 2. In this case AC voltages ae applied upon both of the sleeve 1 and the sleeve 2 by an AC bias power source 11 and a bias power source 10 for coating to make the film thickness of the developer 5 constant. In the stage of nondeveloping, AC bias is applied upon the sleeve 2 alone and DC bias is applied upon the coating sleeve to keep the magnetic brush 5' of the sleeve 5 clean.