A belt driving mechanism includes an endless belt wound around first and second belt supporters, rotated by a driving source, a pressure member that presses against the first belt supporter via the endless belt, a deviation detector to detect a deviation of the endless belt in its thrust direction, a cam mechanism including a rotary cam shaft extending the thrust direction of the endless belt anda first pressure change member of irregular shape, and a controller to changes a rotational position of the first pressure change member according to a detection result by the deviation detector. Thefirst pressure change member has multiple different lengths of radii between a center portion of the cam shaft and its outer circumference and is attached to the cam shaft to press a first end portion of one of the first belt supporter and the pressure member.