PURPOSE: To provide a range for regulating the uneven movement of a flat belt and its accuracy with high stability in a belt drive device which can detect and correct the uneven movement automatically.

CONSTITUTION: An uneven movement detection member 38 whose taper angle changes over the peripheral direction is disposed onto the shaft end part of a roller where a photosensitive belt 6 is wound so as to run, and an uneven movement detection belt 30 is mounted on a part of uneven movement detection member 28. When the photosensitive belt 6 makes uneven movement and is brought into contact with the uneven movement detection member 28, the uneven movement detection belt 30 is wound by the rotation of the uneven movement detection member 28, thus eliminating the initial uneven movement with one shaft end part of the third roller displaced. A sensor which detects the uneven movement of the photosensitive belt 6 is provided, and at the time of detecting the uneven movement with the sensor, the uneven movement detection member 28 is forcibly rotated in the normal and reverse direction according to the uneven movement direction by the rotation of a winding shaft 36 to bring the range of a different taper angle into contact with the photosensitive belt 6.