The bottom plate has a plate-like shape, is arranged to face an outer face of a lower wall, and has a locking portion. The groove member has: a groove-forming portion having a groove opening downward formed therein, a surrounding wall portion, which is connected to the groove-forming portion, and is arranged around the groove-forming portion; and a locked portion, which is connected to the surrounding wall portion, is elastically deformable, and is locked by way of the locking portion of the bottom plate by being elastically deformed. The groove member is supported and fixed by way of the lower wall and the bottom plate.