In accordance with the present invention, in an image forming apparatus of the type developing a latent image formed on an image carrier, which moves at a linear velocity of 400 mm/sec or above, with a developer containing toner grains having a volume mean grain size of 5 &mgr;m to 10 &mgr;m and 60 number percent to 80 number percent of which has a grain size of 5 &mgr;m or below, the developer contains carrier grains having a weight mean grain size of 65 &mgr;m or below. When use is made of toner with a small grain size, the apparatus successfully obviates background contamination, toner scattering and sleeve contamination without regard to operation speed or environmental conditions.