The invention provides a developing device, a process cartridge and an image forming apparatus that allow preventing abrupt variations in toner charge on a developing roller, caused by intrusion, into a nip, of toner accumulated above a regulating member of the developing device. A developing device using a one-component developer includes: a supply roller to which one-component developer is replenished, by natural motion, from a developer storing section. The bottom of the developing device is of a size that allows the supply roller to rotate. The developing device further comprises a filling member for filling a space above the regulating member, in order to prevent the toner located above the regulating member from intruding into a nip formed between the supply roller and a developing roller.