PURPOSE: To enable control in compliance with the curl generated in a sheet by setting curl correcting strength using information on temperature, paper thickness, and the like in addition to image density and humidity in a curl correcting device provided at the sheet discharge part of an image forming device.

CONSTITUTION: A curl correcting device 1 disposed at the photocopy discharge part of an image forming device presses a pressure roller 5 to a belt 2 and curve a curled sheet, passing between both members, suddenly to correct the curl of the sheets. The curl correcting device 1 is provided with mechanism for driving a cam member 10 for pressing the pressure roller 5 to the belt 2 according to the information of pressing force set by a control device. In order to drive the pressure roller 5, information on temperature, paper thickness, and the like in addition to image density and humidity is inputted into the control device so as to display curl correcting performance in compliance with the quantity of curl generated in the sheet according to the information.