Abstract: A developing device using a one-component developer is composed of colored fine synthetic resin toner particles. The device includes a vessel for holding the developer, and a developing roller rotatably provided within the vessel in such a manner that a portion of the roller is exposed therefrom and resiliently pressed against a surface of an electrostatic latent image formation drum. The roller is formed of a conductive open-cell foam rubber material, and a surface thereof is thermally or chemically treated to prevent a penetration of the toner particles to an open-cell foam structure of the developing roller, whereby a softness of the developing roller can be maintained over a long period. The developing device further includes a blade or roller member provided within the vessel and resiliently engaged with the developing roller, for regulating a thickness of the developer layer formed around the developing roller.