To provide a cap capable of being opened without damaging a tamper-proof function without requiring a big torque when a cap body is turned in the cap opening direction by preventing a connecting section between the split ends of a tamper-proof ring from being cut when the cap is attached and the connecting section can be properly cut to allow easy release of the tamper-proof ring when the cap is opened.

A vertical rib 14 which continues from an annular rib 9 is installed on the mouth section outer peripheral surface under the annular rib 9 of the pouring-out port 2. Thus, the fastening flap 7 of the tamper-proof ring 6 which rotates around the mouth section by the rotation of the cap body 3 can get into contact with the vertical rib 14.