Groove formation portions that define insertion grooves are located in two opposing inner sides of an opening of a main cover. Engagement shafts are located on side portions of the sub-cover that oppose the inner sides. The engagement shafts are insertable into the insertion grooves from an outside and have a thickness that is larger than a groove width of the insertion grooves. The groove formation portions are elastic and deformable to allow insertion of the engagement shafts when the engagement shafts are inserted from the outside. The groove formation portions elastically return to their original forms after insertion of the engagement shafts to engage the engagement shafts and restrict separation of the engagement shafts in a reverse direction. The groove formation portions support the sub-cover to allow the sub-cover to open while engaged with the engagement shafts.