PURPOSE: To prevent a develper from being more concentrated along a magnetic field generator, by providing a barrier plate in the direction almost rectangular to the generator in the region not limiting flow of the developer.

CONSTITUTION: Plural partition plates 3d are provided at right angles to the sleeve 3b' of a developing roller 3b and by 20W25mm apart from the surface of the sleeve 3b'. A magnetic flux density on the surface of the roller 3b is about 1,000G, the range of the magnetic field is about 25mm, therefore, the field acts on the plates 3d very little, so they may be made of either of magnetic and nonmagnetic materials, and many plates 3d are arranged at 20W40mm intervals in the direction parallel to that of the sleeve shaft.