A developing apparatus employing a three-pole stationary magnetic roller for use with a uni-component magnetic developer or toner. The apparatus includes a rotational sleeve which accommodates the stationary roller, a mixer disposed in a toner vessel for stirring the toner, and a toner layer thickness regulator disposed upstream in the sleeve rotation direction from the developing position at which the image on a photo-sensitive drum is developed. The magnetic roller has a developing pole located near the developing position, a blade-facing pole facing the toner layer thickness regulator, and a shield pole located downstream in the sleeve rotation direction from the developing position, where the blade-facing pole and the shield pole are of the opposite polarity to the developing pole. The blade-facing pole and the shield pole generate a repulsive magnetic field therebetween, where the toner that has been stirred in the vessel comes in contact with the sleeve.