upstream end and a downstream end of the cleaning nip, the cleaning brush roller 903 contacts a tensioned belt area of the intermediate transfer belt 901 where the intermediate transfer belt 901 is not held in contact with the roller 902. (Hereinafter, the belt wound area where the cleaning brush roller 903 contacts the belt tension area on an upstream side of the nip is referred to as an upstream tensioned nip area.) In this upstream tensioned nip area, because the cleaning brush roller 903 contacts the outer surface of the intermediate transfer belt 901 but the cleaning opposite roller 902 does not contact the inner surface of the intermediate transfer belt 901, the cleaning electrical current described above does not flow sufficiently.
By contrast, in the belt woundarea where the roller 902 contacts the intermediate transfer belt 901 or in the area indicted by the belt wound width W1, the cleaning electrical current can flow well. Specifically, a large amount of electrical current flows more in the vicinity of a center part of the belt wound area where the nip pressure is greatest, than at the ends of the belt wound area. Accordingly, when toner comes to the center part of the belt wound area, toner can be charged to the opposite polarity easily.
Therefore, to obtain good cleaning ability, of the entire area in the belt moving direction in the cleaning nip, it is necessary to transfer substantially all of the residual toner onto the cleaning brush roller 903 in an upstream tensioned nip area