A magnet-brush development process in which the amount of developer powder supplied to a development zone is self-controllable by the rotation of a magnet roll. A permanent magnet member and the shell rotate at different speeds in the same direction in such a manner that developer powder, as a whole, is conveyed on the shell in the direction opposite to the rotation of the magnet roll from a doctor spacing of a developer vessel to an image-bearing material. By controlling the doctor spacing to be larger than the gap between the image-bearing material and the magnet roll, an accumulation of the developer powder occurs on the shell between the doctor spacing and the development zone and is kept to be substantially constant. At the accemulation, an understream of powder moves in the direction of roll-movement towards the powder supply.