A developing device that prevents the formation of agglomerated masses of developing agent in the bottom of the developing agent housing chamber near the developing roller and the supply roller, and that maintains the fluidity of the developing agent in the bottom of the developing agent housing chamber, while minimizing any increase in size of the developing device. The developing device includes an agitator disposed higher than the developing roller, the regulating blade, and the supply roller, and that agitates a non-magnetic one component developing agent within the developing agent housing chamber; and a developing agent ingress prevention member disposed higher than the supply roller, and lower than the agitator, and the side surface or edge portion of the developing agent ingress prevention member near the developing roller is higher than the topmost point of the supply roller, and lower than the lowermost point of the regulating nip formed by the contact of the developing roller and the regulating blade.