PURPOSE: To stabilize a conveyance interval at the time of curling straightening by providing a rectilinear conveyance path between a first and a second conveyance means and curved curling straightening conveyance paths juxtaposed above and below it, and increasing a peripheral speed of the second conveyance means when induction of a sheet into a straightening conveyance path is detected.

CONSTITUTION: A sheet 2 after passing through a fixing device 9 is conveyed to a curling straightening device 12 in a state of upward curling, downward curling, or non-curling according to a kind of the sheet, copying mode and the like. When the sheet 2 is imparted with the upward curling, it is induced to an upward curling straightening conveyance path 16a via a guide member 15a; when imparted with the downward curling, it is induced to a downward curling straightening conveyance path 16b via a guide member 15b; and when without curling, it is rectilinearly advanced to a sheet conveyance path 16c. When the sheet 2 is induced to respective curling straightening conveyance paths 16a, 16b and a tip thereof is detected, a peripheral speed of a second conveyance roller pair 17 is increased to bring a clearance between the sheets 2 having passed a curling straightening device 12 to a constant value, and generation of jamming is thereby prevented.