A belt unit includes a plurality of rollers including a driving roller and a driven roller; a belt being supported by the plurality of rollers; a specified roller; and a shift restraint member restraining the belt from shifting toward one end side of the specified roller. The specified roller is at least one roller of the plurality of rollers. The specified roller has such a shape that an outer diameter thereof on each of both end sides thereof is continuously enlarged toward a central part thereof in an axial direction thereof, and has such a shape that a region where the outer diameter thereof is the largest is shifted onto one end side thereof with respect to the central part thereof in the axial direction thereof within a belt suspension range.