A box and a lid that form a fuel filler hole or a charging socket of a car body, the lid including a lid mounting part, and a box mounting arm including a box engaging pin. An open/close spring is attached to the box mounting arm. The box includes a box body, and an arm housing part extending from the box body and including an arm guide that guides the box mounting arm. The box engaging pin of the box mounting arm of the lid is guided into the arm guide of the box and attached to an arm engaging part of the box, while the open/close spring abuts on a connecting point (a spring mounting portion) between an inclined part and a step part of the arm housing part.