Patent ID: 6985681

Claim:
An image forming apparatus, in which a toner image formed on a photosensitive member is transferred onto a transfer paper through an intermediate transfer medium which comprises a plurality of layers including a conductive layer and which moves from a primary transfer part to a secondary transfer part by revolution, said apparatus comprising: primary transfer means which primarily transfers said toner image from said photosensitive member onto said intermediate transfer medium in said primary transfer part by applying a primary transfer bias which is determined in advance upon said conductive layer of said intermediate transfer medium; secondary transfer means which secondarily transfers said toner image now on said intermediate transfer medium onto a transfer paper in said secondary transfer part; and bias control means which changes an output value of said primary transfer bias in accordance with a predetermined bias change condition, wherein said primary transfer part and said secondary transfer part are so arranged that a primary transfer by said primary transfer means and a secondary transfer by said secondary transfer means can be executed at the same time, said bias control means changes said output value when the secondary transfer is not ongoing, and a timing of the change of said output value by said bias control means is set in advance such that changing of said output value is completed while a non-image area, in which there is no toner image transferred on said intermediate transfer medium, is passing said primary transfer part.