A developing device using a one-component developer composed of colored fine synthetic resin toner particles, which device comprises a vessel for holding the developer, and a developing roller rotatably provided within the vessel, a portion thereof being exposed therefrom and resiliently pressed against a surface of a photosensitive drum. The toner particles are held by the surface of the developing roller to form a developer layer therearound, and are carried to the surface of the image formation drum for development of an electrostatic latent image formed thereon. The developing device further comprises a blade member provided within the vessel and resiliently engaged with the developing roller for regulating a thickness of the developer layer formed therearound. The blade member is constituted such that a proper regulation of the developer layer can be always ensured.