A developing device is used to develop an electrostatic latent image with a one-component type developer, the latent image being formed on a photosensitive body. The device has a developing roller made of a conductive rubber, and a conductive sponge roller engaged with the developing roller to feed the developer to the developing roller. The rollers are rotated in the same direction, and the developer fed to a nip between the rollers is subjected to triboelectrification by the rollers, whereby the developer is electrically charged. The charged developer is electrostatically adhered to the surface of the developing roller due to an electrostatic image force acting therebetween. The sponge roller has a friction plate associated therewith, for effectively and additionally charging the developer. Also, the developing roller has a developer-removing plate associated therewith, for removing residual developer, not used for development of latent images, from the developing roller.