A developing method of the present invention develops a latent image formed on an image carrier with a two-ingredient type developer, which consists of toner grains and carrier grains retaining the toner grains thereon, deposited on a developer carrier in the form of brush chains. Assume that the brush chains on the developer carrier has a height of h as measured at a zero field point where the magnetic field formed by a main magnet is zero, and that a gap for development that is the shortest distance between the developer carrier and the image carrier is a, then a and h are equal to each other. Image quality is estimated by varying the combination of an angle .theta. between the gap and the zero field point, the outside diameter of the image carrier and the outside diameter of the developer carrier and the height h. A desirable or an optimum range of the results of estimation are selected to set various conditions.