In a bottle cap (10), a bottom cover member (40) is separated from an accommodation portion (22) while an outer peripheral wall (44) of an annular groove is pressed against a lower end surface (39) of an outer cylinder member (30) when a cap portion (26) that is threadedly mounted on the outer cylinder member (30) threadedly advances in an opening direction with respect to the outer cylinder member (30). The lower end surface (39) of the outer cylinder member (30) is inclined downward and outward. A ridge (45) is integrally formed with an upper edge of the outer peripheral wall (44) of the annular groove by the same material as that of the outer peripheral wall (44), where the ridge (45) is pressed against the lower end surface (39) of the outer cylinder member (30) while the cap portion (26) is threadedly mounted on the outer cylinder member (30).