PROBLEM TO BE SOLVED: To provide an image forming apparatus which can efficiently transmit a driving force to respective devices disposed near a photoreceptor drum while achieving space-saving, and also, prevent banding and positional deviation, thereby, obtain a high quality image. SOLUTION: A holding member 114 is provided with a positioning part 115 for positioning a photoreceptor drum unit 103 and an attitude deciding part 116 for deciding the attitude of the drum unit 103. A positioning part 105 and an attitude deciding part 106 located on the drum unit side are fitted to the positioning part 115 and the attitude deciding part 116, then, the position and attitude of the drum unit 103 are simultaneously decided. Thereby, the position and attitude of a developing sleeve included in the drum unit 103 are also decided. COPYRIGHT: (C)2009,JPO&INPIT