PURPOSE: To miniaturize the device even when a multiphase armature is used by linearly disposing multiphase armature coils under a developer container, which is provided under an image carrier, so that they are shifted from each other as much as a prescribed electrical angle between the phases, and generating a traveling-wave magnetic field on the developer container.

CONSTITUTION: A developing part 10 is arranged under and close to a photosensitive drum 1; under a toner moving container 11 which is nonmagnetic and is in the form of a box, the first and second armature coils 12 and 13 are linearly disposed along the container 11. The traveling-wave magnetic field of the linear type is generated on the toner moving container 11 by simultaneously energizing the armature coils 12 and 13 with their corresponding AC currents which are shifted from each other as much as a prescribed amount in terms of phase. Consequently, toner moves horizontally from a supply part 11a, via a toner transfer part 11b, to a toner circulating part 11c, and is supplied to the photosensitive drum 1 at the toner transfer part 11b closest to the photosensitive drum 1; and thus, an electrostatic latent image on the photosensitive drum 1 is developed.