A developing device includes a developing sleeve for supplying toner particles to a latent image area on a peripheral surface of a photoconductive drum while rotating about an axis. The developing sleeve has a permanent magnet therein. Two magnetic members are provided in the developing device and face opposite ends of a peripheral surface of the developing sleeve at a predetermined distance from the developing sleeve. The magnetic members have arc-shaped inner surfaces. A blade is spaced a predetermined distance from a central portion of the peripheral surface of the developing sleeve. The blade adjusts the amount of toner particles supplied to the photoconductive drum. The gap between the arc-shaped inner surface of the magnetic member and the peripheral surface of the developing sleeve gradually increases from upstream to downstream in a rotational direction of the developing sleeve.