A developer for an image forming apparatus includes a developing casing having a plurality of toner chambers to contain toner, an agitator having a resilient wing and rotatably disposed in the toner chamber for agitating and transferring toner, a toner supplying roller rotatably disposed adjacent to one of the toner chambers of the developing casing, a developing roller for transferring and attaching the toner supplied from the toner supplying roller to a photoconductive medium, and a partition disposed in the developing casing to separate the two toner chamber. The partition has a plurality of slits at a center potion to transfer toner from one toner chamber to another toner chamber therethrough, while the agitator rotates. Also, the partition has a rib sloped upward at a predetermined angle from a lower portion of the slit. While the agitator rotates, the resilient wing of the agitator resiliently contacts with the rib so as to splash the toner upward. The toner being splashed then drops due to gravity and moves through the slit.