To provide a belt driving device reducing a relative difference of a belt winding angle with respect to a steering roller in the range of tilt adjustment of the steering roller, and then accurately controlling the biasing of an endless belt, and also to provide an image forming apparatus.

The steering roller, a first suspension roller and a second suspension roller are disposed so that a distance relation between a roller center-to-center distance from the steering roller to the first suspension roller disposed in just upstream of the steering roller and a roller center-to-center distance from the steering roller to the second suspension roller disposed in just downstream of the steering roller may establish that the ratio of the other becomes 1.2 when shorter one of both distances is regarded as a reference. When the tilt amount of the steering roller is varied at equal intervals, the winding angle of the endless belt with respect to the steering roller, varying in accordance with the tilt amount of the steering roller is made to have a relative difference of 1% in the range of the adjustment.