Abstract: A developer unit includes a porous conductive resilient member which rotates while partly contacting a developer holder to charge a non-magnetic single component developer and to supply it to the surface of the developer holder. A conductive constraining member forms a uniform thin layer of developer on the developer holder and also charges the developer to a given level. A developing bias voltage is applied to the developer holder via a rotatable fibrous charging member to cause the developer to fly to an image area of an electrostatic latent image formed on a latent image carrier which is disposed in opposing relationship with the developer holder with a gap therebetween. The bias voltage is also applied to the constraining member and to the charging member.