A belt device includes a belt configured to rotate, a plurality of stretch rollers configured to stretch the belt wound around the plurality of stretch rollers, and a belt alignment mechanism configured to align the belt. At least one of the plurality of stretch rollers has a diameter at a center greater than diameters at both ends in an axial direction of the at least one of the plurality of stretch rollers. The diameter at the center and the diameters at both ends are different from a diameter of the other of the plurality of stretch rollers.