an image forming method for operating a moving image - bearing member bearing an electrostatic latent image and a plurality of developing units disposed near the image - bearing member and being selectively operable is described . the color image forming method develops the electrostatic latent image by using a magnetic developer brush formed on the surface of a developer support and conveyance means , and includes the steps of : providing a rotating cylinder for supporting and conveying the magnetic developer , the cylinder including a permanent magnet with a plurality of magnetic poles on the cylinder surface ; supplying a magnetic developer containing non - magnetic color toner for color image formation ; attracting and conveying a layer of magnetic developer having a smaller thickness than the gap between the image - bearing member and the developer support and conveyance cylinder on the cylinder surface ; and applying in the developing region an alternate electric field with an ac bias superimposed on a dc bias between the image - bearing member and the magnetic developer for the developing of an electrostatic latent image on the image - bearing member . the method also includes a surface magnetic flux density ranging from 50 to 1200 g , and a value of h , defined as πd · v p /, of 2 or less wherein v p is the moving speed of the image - bearer , and d , m , and v m are the outer diameter , the number of magnetic poles , and the peripheral speed of the permanent magnet , respectively .