A de-curling device de-curls a sheet and includes an endless belt, a de-curling roller, a nip width adjusting mechanism, and a belt tension adjusting mechanism. The endless belt is looped around a pair of supporting rollers. The de-curling roller is provided between the pair of supporting rollers and has a first outer circumferential surface pressed against a second outer circumferential surface of the endless belt to form a nip portion at which the endless belt curves along the first outer circumferential surface, the de-curling roller being configured to de-curl the sheet passing through the nip portion. The nip width adjusting mechanism adjusts the nip width by moving the de-curling roller in a direction intersecting the second outer circumferential surface of the endless belt. The belt tension adjusting mechanism adjusts the tension of the endless belt according to the nip width adjusted by the nip width adjusting mechanism.