A belt meandering preventing device includes a belt displacement detection unit detecting the amount of displacement in a belt width direction of an endless belt rotatably stretched over support parts; and a belt meandering correction unit correcting the displacement in the belt width direction of the endless belt based on the amount of displacement detected by the belt displacement detection unit. The belt displacement detection unit includes a moving part moving in association with the displacement of the endless belt or an edge of the endless belt in the belt width direction and optical sensors outputting signals with output levels corresponding to the proportions of the moving part in optical paths of the optical sensors. The optical sensors are arranged such that the output levels of the optical sensors change as the endless belt is displaced in the belt width direction in a predetermined high-resolution detection range.