There is described an image forming apparatus for forming a high-density image by using the reversal developing method and suppressing an occurrence of irregularities. The apparatus includes an image bearing member, a latent image forming device to form an electrostatic latent image, a magnetic field generating device to generate a magnetic field and a developing device to develop the electrostatic latent image so as to form a toner image on the image bearing member. At a developing region, the developer bearing member moves in a direction opposite to a moving direction of the image bearing member and the magnetic field generating device has a developing pole to form a magnetic brush for developing the electrostatic latent image. Further, an outer diameter R (mm) of the developer bearing member and a magnetic flux density Br1 (mT) of the developing pole fulfill the relationship of Br1≧R×4.5.