Abstract: An image forming apparatus (1) comprises a positioning mechanism (53) which displaces a first bearing (63) bearing a first end (62a) of a rotation shaft (62) of a backup roller (52) so that a rotation axis of the backup roller (52) takes a predetermined angle with respect to a rotation axis of a hot roller (51). When a thick envelope (E1) is printed, it is passed between the hot roller (51) and the backup roller (52) in a portion where a clearance between the hot roller (51) and the backup roller (52) is expanded in the vicinity of the first end (62a). When a thin envelope (E2) is printed, it is passed between the hot roller (51) and the backup roller (52) in a portion where a clearance between the hot roller (51) and the backup roller (52) is a little expanded in the vicinity of a second end (62b). Thereby, images can be formed properly on both of the thick envelope and the thin envelope without occurrence of corrugation or fixing failure of toner image on a surface of the envelope.