A developing unit comprises a developing portion, a first developer storage portion and a partitioning wall. The developing portion includes a rotatable developer carrier, a rotatable supply member configured to supply the developer to the developer carrier, a regulating member contacting with the developer carrier to regulate the thickness of the developer on the developer carrier, and a developer feeding member arranged below the regulating member and configured to feed the developer. The partitioning wall is configured to partition between the developing portion and the first developer storage portion. The partitioning wall has a supply port for supplying the developer from the first developer storage portion to the developing portion. The regulating member is contacting with the developer carrier at a contact position located above an upper level of the developer in the developing portion.