A semimanufactured product of a stopper (31) includes: a plate-shaped bridging portion (33); a first curved portion (35) extending from one end portion of the bridging portion (33) integrally; and a second curved portion (37) extending from the other end portion of the bridging portion (33) integrally, and an entirety of the semimanufactured product is formed in a C-shape. The first curved portion (35) includes a first locking part (41) at the end portion. The second curved portion (37) includes a second locking part (51) that engages with the first locking part (41) at the end portion. The first locking part (41) includes a convex-shaped convex portion (43) and a pair of receiving portions (45) positioned so as to sandwich the convex portion (43). The second locking part (51) includes: a concave-shaped concave portion (53) which can accommodate the convex portion 43; and a pair of projecting portions (55) positioned so as to sandwich the concave portion (53). Each of the pair of projecting portions (55) includes a protruding portion (57) which protrudes from a tip portion of the projecting portion (55) toward an opening (54) of the concave portion (53). The convex portion (43) includes constricted portions (44) in base end portions of the convex portion.
