Patent ID: 7826781

Claim:
An image forming apparatus, comprising: a photoconductive member on which a latent image is to be formed; a developing roller for developing the latent image formed on the photoconductive member by a first bias; a magnetic roller for forming a magnetic brush thereon with a two-component developer containing a carrier and a toner and forming a thin toner layer on the developing roller by a second bias; and a bias applying device including a first power supply for generating biases in which a first alternating-current bias is superimposed with a first direct-current bias and a second power supply for generating biases in which a second alternating-current bias is superimposed with a second direct-current bias, and adapted to apply the biases to the developing roller and the magnetic roller, wherein: a bias of the first power supply is applied as the first bias to the developing roller, a superimposed bias of the bias of the first power supply and that of the second power supply is applied as the second bias to the magnetic roller, the first bias includes a first alternating-current bias in the form of a rectangular wave; and if CV denotes coefficient of variation calculated as (standard deviation in the number particle size distribution/number average particle diameter)×100 Dt denotes the volume average particle diameter of the toner, pv denotes the intrinsic resistance value of a developing roller surface, Ra denotes the arithmetic average roughness of the developing roller surface and D 1 denotes the duty ratio of the first alternating-current bias defined by the equation D 1 =(a 1 /(a 1 +a 2 ))×100 where a 1 is a positive period of the rectangular wave and a 2 is a negative period of the rectangular wave, the following relationships are satisfied in the case of calculating the duty ratio D 1 using an application period of a voltage in a direction to transfer the toner from the developing roller toward the photoconductive member as a positive period: a CV value in the number particle size distribution of the toner is 25% or smaller, 4 μm≦Dt≦7 μm, 10 5 Ω·cm≦pv≦10 9 Ω·cm, 0.4 μm≦Ra≦1.5 μm, and 35%≦D 1 ≦75%.