Patent ID: 6873814

Claim:
In a developing device comprising a developer carrier whose surface moves with a developer consisting of toner grains and magnetic grains deposited thereon, where an adjacent image carrier whose surface moves with a latent image formed thereon, and a magnetic pole facing a developing zone where said developer carrier and said image carrier face each other, causing said developer to rise in a form of a magnet brush on said developer carrier in said developing zone due to a force of said magnetic pole, and causing said surface of said developer carrier to move in a same direction as, but at a higher linear velocity than, said surface of said image carrier in said developing zone to thereby cause said magnet brush to rub said surface of said image carrier and develop said latent image, the toner grains comprise magnetic grains consisting of at least binder resin and a magnetic substance, flux density formed by said magnetic pole in a direction normal to the surface of said developer carrier outside of said surface has an attenuation ratio of 50 or above, the magnetic toner grains have a weight mean grain size of 6.0 μm to 8.0 μm, and the magnetic toner grains having a grain size of 5 μm or below are contained in the developer by 40% to 80% and have magnetization strength of 10 emu/g to 25 emu/g in a magnetic field of 5 kOe or magnetization strength of 7 emu/g to 20 emu/g in a magnetic field of 1 kOe.