To reduce the likelihood of erroneous detection resulting from improper engagement of an engaging part with an engaged part, a fixing member is made to have a recess formed therein and is provided with a plurality of conduction paths that are electrically insulated from each other and have portions that are exposed at a plurality of positions including a first position inside the recess and in the vicinity of the innermost part of the recess and a second position inside the recess and not in the vicinity of the innermost part of the recess and other portions that are exposed outside of the recess, and a prescribed member touching the first position and second position can push the area around the opening of the recess.