operation of the photosensitive member positioning and rotation driving mechanism first the positioning operation will be described when the image forming unit 3 has been shifted to the image forming position 10 in the apparatus main body 1 the driving pin 61 of the photosensitive member positioning and rotation driving mechanism 60 and the output axis 70 having the output driving gear 71 moves in the thrust direction by the thrust bearing 69 at the engaging position the tip taper portion 75 of the output axis 70 is positioned corresponding to a first hole portion 40 a of the first flange 40 at the same time the first coupling portion 40 b of the first flange 40 engages the driving pin 61 input axis 81 having a driving pin 82 of the rotation driving mechanism 80 provided on the left side wall 1 l of the apparatus main body moves in the thrust direction by the thrust bearing 86 in synchronization with the motion of the output axis 70 and tip taper portion 85 of the input axis 80 is positioned corresponding to the second taper hole portion 41 a of the second flange 41 at the engaging position and at the same time the second coupling portion 41 b of the.