An image forming apparatus includes an image forming unit which has an image bearing member and which is detachably attachable to the image forming apparatus, a guide section for guiding an insertion operation of the image forming unit, a movable locking member for locking the attached image forming unit from moving along a guide direction, a moving member which moves when the moving member comes into contact with the image forming unit which is guided and inserted by the guide section, and a locking member moving portion which starts moving the locking member when a moving amount of the moving member becomes great.