To provide a curl straightening device capable of preventing the curl straightening amount from being inappropriate even when the curl of a recording material with oil deposited thereon is continuously straightened.

Intrusion of hard rollers 303, 304 in soft rollers 305, 306 is set according to the combination of the toner carrying amount and the basic weight of recording materials. When the total number of the curl-straightened recording materials exceeds the predetermined value A, the control amount of the intrusion to be set to the same combination of the toner carrying amount with the basic weight of the recording materials is reduced by one stage. Thus, the swollen amount of the soft rollers 303, 304 caused by the absorption of oil deposited on the recording materials by a fixing device in the soft rollers 303, 304 is canceled to maintain the curl straightening capacity of a decurler 301A.