US Patent 7333759 Image forming apparatus

A transfer roller drive unit sets an all-separate mode after setting a full-color mode, sets a monochrome mode after setting the all-separate mode, and sets a full-color mode after setting the monochrome mode. Namely, a transfer belt 25 comes into contact with the photosensitive drum 24 first, and then the photosensitive drums 21, 22 and 23. This pattern is repeated. The transfer belt 25 does not come into contact with the photosensitive drums 21, 22, 23 and 24 at a time.