The present invetnion relates to a decelerator, which is arranged in image forming devcie, such as a copy, a fax machine and a printer, and is used for reducing a speed of rotation of a drive source for an image bearing member and transmitting in the image bearing member, the decelerator uses friction as driving transmission, at the same time inhibits the change of the rotation speed, and reallyperforms driving transmission, the present invention also relates to an image bearing member driving device provided with the decelerator and used for performing rotation driving, and an image forming device. The device comprises a drive shaft 86 rotated by the drive source 81, and a circumferential surface 85a connected with the drive shaft 86, a drive transmission component 85 rotated by the drive shaft 86, and a decelerator 82 transmitted in an image bearing member 20 after being driven by the drive source 81, an alignment regulator 87a for regulating the offset of the drive shaft 86 in the rotating direction of the drive transmission component 85, and a pressure regulator 88 for pressing the drive shaft 86 against a center of rotation of the drive transmission member 85.