In an image forming apparatus, a developing device is disposed adjacent to a belt-like photosensitive member. A rotary type developing device has a hollow cylindrical casing rotatable around a shaft having a central portion for rotation. The developing device includes a casing which is rotated in a predetermined direction, and four partitions which divide the casing to make four developing units which extend radially outward from the shaft. A toner storing section or hopper is provided on one side of the developing device, i.e., on a front side of the image forming apparatus, and a toner cartridge is detachable from a top of the hopper, and the toner cartridge is exchanged when an end signal of the toner is detected by a sensor in the casing. The toner hopper is filled by new toner from the toner cartridge, and the toner is carried by conveyor members, which control an amount of the toner in the casing, and a supply roller supplies the toner to a developing roller. Therefore, the device supplies a required amount of toner to a toner supply roller and a developing roller at all times. This provides an image forming apparatus with a great freedom of layout which needs a minimum sectional space relative to a photoconductive element and which, therefore, has a miniature configuration. Also, if amounts of transport of first and second conveyor members are adequately selected, a uniform developer distribution in a longitudinal direction without resorting to a complicated control is insured.