Catching grooves 2d, 2e are formed in a turning space 2f of a catching protrusion bar 2b, engagement projections 3c, 3d are formed on a standing part 3b of a catching member 3a, and the standing part 3b is inserted into the turning space 2f, so that the engagement projections 3c, 3d are respectively engaged with the catching grooves 2d, 2e by the own weight of the catching member 3a.