center adjusting unit 60 of the belt member 1 as explained below fig4 and 5 show an enlarged center adjusting unit 60 of the belt member 1 the belt surface is omitted in the present embodiment as described above the belt tensioning device 50 has the belt member 1 for transferring a toner image on the photosensitive drum 111 onto the transferring material p a first supporting member driven roller 3 or tension roller 4 for supporting the belt member 1 and a second supporting member drive roller 2 capable of supporting the belt member 1 and changing an angle with respect to the first supporting member thus the belt supporting device 50 has contact members 13 a 13 b which are drive receiving means constructed such that they can come into contact with the belt member 1 and a contact state with the belt member 1 changes and in response to a contact state between these contact members 13 a 13 b and the belt member 1 an angle of the second supporting member drive roller 2 to the first supporting member driven roller 3 or tension roller 4 is adapted to change thereby the belt member 1 moves in a direction indicated by.