An extension kit includes a case, a locking lever, and a first elastic member. The case has an interior space which extends in a first direction. The locking lever includes a lever portion which is rotatable about a first rotation axis extending in a second direction intersecting the first direction, a plate portion which extends from the lever portion and which is exposed at the case, an arm portion which extends in the second direction from the plate portion, and a first locking portion which protrudes from the arm portion. The first elastic member is provided in the interior space of the case and elastically connects the case and the locking lever in the interior space of the case.