A photoreceptor drum driving mechanism includes a drum drive gear provided at a leading end of a drive shaft, which is in mesh with an internal gear mounted to a photoreceptor drum and a roller coaxially formed with the drive shaft. The mechanism further includes a circular arc surface in contact with the roller in a deflecting direction of the drive shaft. As a result, the drive shaft of the drum drive gear can be prevented from being deformed in a pressure angle direction of the drum drive gear. As this eliminates irregularities in rotations of the photoreceptor drum, a distortion of an image in an image forming process or in a transferring process can be prevented.