Automatic toner supplying device

An automatic toner supplying device for use in an electrophotographic copying machine which comprises a developing roller rotatably disposed in a developer housing adjacent to a photoreceptor surface of a photoreceptor drum, through an opening formed in said developer housing, magnet means enclosed in a rotatable outer cylinder of said developing roller for formation of magnetic brush bristles of developing material consisting of carrier beads and toner particles on an outer periphery of said outer cylinder, said magnetic brush bristles being adapted to rub against a latent image formed portion on said photoreceptor surface for developing said latent image into a visible toner powder image, a toner supply roller with which carrier beads are adhered rotatably disposed at a lower portion of a toner supply tank incorporated in said developer housing and on which magnetic brush bristles are also formed through magnet means enclosed in said toner supply roller, an offset drum of electrically conductive material rotatably disposed between said developing roller and said toner supply roller for transferring toner particles on said brush bristles of said toner supply roller onto said brush bristles of said developing roller and an electrical control circuit connected to said developing roller, said toner supplying roller and said offset drum, said toner particles consumed for said development being proportional, in amount thereof, to potential variations of said brush bristles which contact said photoreceptor surface during rotation of said photoreceptor drum and said developing roller, said potential variations being utilized as a toner replenishing signal for applying a bias voltage corresponding to said toner replenishing signal, through said control circuit, to said brush bristles formed on said toner supply roller, which brush bristles are adapted to supply, through said offset drum, an amount of toner particles approximately equal to said amount of said toner particles consumed for said developing, to said brush bristles of said developing roller.