In an image forming apparatus including a rotary member (15) to which a plurality of developing cartridges (B, Y, M, C) (16) are detachably mountable, a user can rotate the rotary member with an easy operation to access the plurality of developing cartridges. A pivotable unit (63) is contactable and separable from a driving train for the rotary member (15). By applying a certain braking force to a pivotal gear (69), the pivotal gear (69) is moved to a position at which the pivotal gear (69) is held in contact with the driving train by a user's operational force. In association with the contact movement of the pivotable unit (63), a solenoid (57) performing a positional control of the rotary member (15) is released.