A curl straightening device includes an endless belt, a curl straightening roller, a nip width adjusting mechanism, and a guide member. The curl straightening roller makes pressure contact with an outer peripheral surface of the endless belt, forming a nip portion, and straightens curl of the sheet which passes through the nip portion. The nip width adjusting mechanism moves the curl straightening roller in a direction away from or close to the endless belt so as to change a nip width. The mechanism includes a supporting holder of the curl straightening roller, and a rotationally moving portion that rotationally moves the roller supporting holder for moving the curl straightening roller. The guide member includes a pair of hanging members that are hung from both ends of the curl straightening roller so as to keep the guide member in constant position and posture relative to the curl straightening roller.