Patent ID: 8055169

Claim:
A developing device comprising: an image supporting member to be driven by a drive motor to rotate in a first direction; and a developer supporting member to be driven by the drive motor to rotate in a second direction opposite to the first direction, said developer supporting member abutting against the image supporting member for developing a static latent image on the image supporting member, said developer supporting member having the Asker C hardness of 65 degree to 83 degree, said developer supporting member being arranged so that, in a state that a predetermined film member is disposed between the image supporting member and the developer supporting member at a position away from an end portion of the developer supporting member by 10 mm, the film member is extended with a tensional force N (N) in a tangential direction relative to the first direction and the second direction to slide against the image supporting member and the developer supporting member when the image supporting member and the developer supporting member rotate, said tensional force N having a relationship as follows: 0.003(mm 2 /N )≦( A×B× exp(0.32 ×F −16))/ N ≦11.6(mm 2 /N ) wherein A (mm) is a difference between an outer diameter of a center portion of the developer supporting member and an outer diameter of an end portion of the developer supporting member and is greater than zero (A>0); B (mm) is a wobble amount of the end portion of the developer supporting member; and F (degree) is an Asker C hardness of the developer supporting member.