A stopper storage case has openings and drill blade insertion paths. The openings are large enough to allow a spring-like engagement portion of the stopper to pass. Each drill blade insertion path is large enough to allow a drill blade to pass. Concave parts extend from the respective openings to the drill blade insertion paths and can store stoppers of different lengths. Each of the concave parts has a protrusion to be engaged in a groove in the outer peripheral surface of the stopper. Stopper holding overhangs formed at the top of each concave part are elastic enough not to interfere with insertion of the stopper from above into the concave part and removal of the stopper from inside the concave part.
