US Patent 7773923 Developing apparatus and image forming apparatus

A developing apparatus in which a toner has a volume-average particle diameter R that is in a range of 4.0 μm≦R≦6.2 μm, a developing roller has an arithmetic mean roughness Ra (μm) that is not more than 0.10 times the volume-average particle diameter R of the toner, a voltage applied to a supply roller is of negative polarity, that is, a normal charging polarity of the toner, triboelectrification polarity of the supply roller with respect to the toner is negative, that is, of the normal charging polarity of the toner, and an abutment width S in which the developing roller and the supply roller abut each other is in a range of 2.5 mm≦S≦5.5 mm.