A multipolar magnet is arranged inside a developer bearing member. A partitioning wall is arranged such that an apex position thereof is located between the developer bearing member and a first conveying member, and is extended up to a position which is located below an upper end position of a zone. A plurality of concaved portions is formed on the surface of the developer bearing member at an interval. Each of the plurality of concaved portions has such an opening shape that a maximum diameter of an inscribed circle is equal to or greater than a diameter of an average grain size of a carrier, and is formed such that the carrier with the average grain size is able to enter each of the concaved portions by a depth that is equal to or greater than a radius thereof.

A developing device includes a developing container, a feeding member, a developer carrying member, and a collecting device. A coverage which is a percentage of coating of surfaces of the carrier particles with the toner particles is 100% or more and 200% or less. The developer carrying member has a plurality of recessed portions formed on a surface thereof so that at least the toner particles having an average particle size are contactable with inner surfaces of the recessed portions, and the carrier particles having an average particle size are not contactable with the inner surfaces of the recessed portions. The recessed portions are formed so that not less than half of the toner particles having the average particle size are exposed from the recessed portions when the toner particles having the average particle size enter the recessed portions.

A developing unit includes a developing container, a developing sleeve, a developer regulating member, and a magnetic field generating portion. The magnetic field generating portion includes a drawing-up pole and a cut pole. In addition, in a peak-to-peak area, the magnetic field generating portion includes at least one of an Fr flat area in which Fr is substantially constant and an Fr attenuation area in which Fr attenuates toward the cut pole side from the drawing-up pole side, and is configured that F is oriented toward the same direction as a direction of rotation of the developing sleeve in the entire peak-to-peak area.

A developing device includes a developing container, a developing sleeve, a magnet and grooves provided at a surface of the sleeve and formed along a direction crossing a circumferential direction of the sleeve. In a cross-section, each of the grooves is formed by a flat bottom portion contacting a carrier particle and a pair of side surface portions provided in both sides of the flat bottom portion with respect to the circumferential direction of the sleeve and satisfies the following relationship:

r<w<2r,

2r<L, and

r/2s<2r.

In the above, r is a volume-average particle size of the carrier particles, w is a length of the flat bottom portion, L is a width between the side surface portions at the surface of the sleeve, and s is a depth of each of the grooves.

A developing apparatus includes a developing container configured to accommodate a developer containing toner and carrier, a developing sleeve rotatably supported by the developing container and configured to carry the developer from the developing container, and a magnet provided in the developing sleeve and having a plurality of magnetic poles arranged in a circumferential direction. In addition, a regulating member is provided opposite to the developing sleeve with a predetermined gap therebetween and configured to regulate a layer thickness of the developer carried on the developing sleeve. The magnetic poles include a regulation pole disposed opposite to the regulating member.

A developing device includes a first rotatable member; a first magnet including a first magnetic pole, a second magnetic pole, and a third magnetic pole identical in polarity to the second magnetic pole; a second rotatable member; and a second magnet including a fourth magnetic pole different in polarity from and closest to the second magnetic pole, a fifth magnetic pole, and a sixth magnetic pole identical in polarity to the fifth magnetic pole. In a rotational direction of the second rotatable member, a maximum magnetic flux density position of the fourth magnetic pole of the second magnet in a normal direction relative to an outer peripheral surface of the second rotatable member is positioned downstream of the closest position of the second rotatable member to the first rotatable member on the outer peripheral surface of the second rotatable member, and is positioned upstream of the fifth magnetic pole.

Disclosed is a development apparatus that includes a first roller to be supplied with developer accommodated in a first chamber, a first magnet, a second roller to which the developer is delivered from the first roller through a magnetic field generated by the first magnet, which is arranged to face the first roller, the second roller bearing and conveying the developer to collect the developer in a second chamber after the electrostatic latent image is developed, and a second magnet, in a case where ten-point average roughness of an outer circumferential surface of the first roller is Rz1 and ten-point average roughness of an outer circumferential surface of the second roller is Rz2, 0.35Rz1Rz2 and 7 mRz115 m.

An outer sleeve of a magnetic roll for a dual component development electrophotographic image forming device according to one example embodiment includes a series of grooves in an outer surface of the outer sleeve. The grooves extend along an axial length of the sleeve and are spaced circumferentially from each other around the outer sleeve. The outer sleeve has a diameter of between 15 mm and 30 mm, inclusive. The grooves are present on the outer surface of the outer sleeve at a groove density of at least 1.91 grooves/mm of the circumference of the outer surface of the outer sleeve and a total indicated runout of the outer sleeve is 0.05 mm or less.

A developing device includes a cylindrical developing roller, a regulation member, a magnetic member, and a synthetic resin member. The regulation member includes a first opposing surface facing an outer circumferential surface of the developing roller. The magnetic member is arranged on a side surface of the regulation member. The synthetic resin member is arranged on a side surface of the magnetic member. The synthetic member includes a second opposing surface facing the outer circumferential surface of the developing roller. The second opposing surface is tilted at 40 or more but 60 or less with respect to the first opposing surface to be increasingly distant from the outer circumferential surface of the developing roller toward an upstream side in a rotating direction of the developing roller. A solubility parameter of a material of the synthetic resin member is 23 (J/cm.sup.3).sup.1/2 or more. Magnetic toner contains toner and resin fine particles.

A developing device includes a developing container, first and second rotatable developing members, a first magnet provided inside the first rotatable developing member, and a second magnet provided inside the second rotatable developing member. A rotational axis of the second rotatable developing member is positioned above a rotational axis of the first rotatable developing member. The developer is delivered from the first rotatable developing member to the second rotatable developing member against a gravitational direction by a magnetic field generated between the first magnet and the second magnet. On an outer peripheral surface of the first rotatable developing member, a plurality of first grooves are formed along a circumferential direction of the first rotatable developing member. On an outer peripheral surface of the second rotatable developing member, a plurality of second groves are formed along a circumferential direction of the second rotatable developing member.