A developing unit comprising: a developing portion including: a rotatable developer carrier configured to carry developer; a rotatable supply member arranged below the developer carrier and configured to supply the developer to the developer carrier; a regulating member configured to come into sliding contact with the rotatable developer carrier to regulate the thickness of the developer on the rotatable developer carrier; and a developer feeding member arranged below the regulating member and configured to feed the developer in a direction of a rotation axis of the rotatable supply member; a first developer storage portion configured to accommodate the developer and arranged adjacent to the developing portion; and a partitioning wall configured to partition between the developing portion and the first developer storage portion, the partitioning wall having a supply port for supplying the developer from the first developer storage portion to the developing portion, and further having a return port for returning the developer from the developing portion to the first developer storage portion, wherein the regulating member is contacting with the rotatable developer carrier at a contact position located above an upper level of the developer in the developing portion and wherein the first developer storage portion includes a first agitating member configured to transport the developer to the supply port by rotating in sliding contact with a wall which forms the first developer storage portion, characterized in that the partitioning wall has a shape depressed toward the interior of the first developer storage portion, the supply port is formed on the partitioning wall on an upstream side of a near point, which is closest to a rotation axis of the first agitating member, in a rotation direction of the first agitating member, and the return port (71 C) is formed on the partitioning wall on a downstream side of the near point in the rotation direction of the first agitating member. a developing portion including: a rotatable developer carrier configured to carry developer; a rotatable supply member arranged below the developer carrier and configured to supply the developer to the developer carrier; a regulating member configured to come into sliding contact with the rotatable developer carrier to regulate the thickness of the developer on the rotatable developer carrier; and a developer feeding member arranged below the regulating member and configured to feed the developer in a direction of a rotation axis of the rotatable supply member; a rotatable developer carrier configured to carry developer; a rotatable supply member arranged below the developer carrier and configured to supply the developer to the developer carrier; a regulating member configured to come into sliding contact with the rotatable developer carrier to regulate the thickness of the developer on the rotatable developer carrier; and a developer feeding member arranged below the regulating member and configured to feed the developer in a direction of a rotation axis of the rotatable supply member; a first developer storage portion configured to accommodate the developer and arranged adjacent to the developing portion; and a partitioning wall configured to partition between the developing portion and the first developer storage portion, the partitioning wall having a supply port for supplying the developer from the first developer storage portion to the developing portion, and further having a return port for returning the developer from the developing portion to the first developer storage portion, wherein the regulating member is contacting with the rotatable developer carrier at a contact position located above an upper level of the developer in the developing portion and