A sheet straightening unit including a device for pushing a depressing roll against an endless belt, so that a curled sheet is straightened when passing between the endless belt and the depressing roll pushed against the latter. The depressing roll is provided with an amount-of-depression adjusting mechanism which includes a cam member and a cam follower member engaged with the former, in such a manner that the cam member is turned according to the kind and image density of the curled sheet, thereby to most suitably determine the extent to which depressing roll is pushed against the endless belt.