To provide a structure of a cap with a tamperproof function and a container mouth part in which a tamperproof ring is not detached from the mouth part of the container, a die structure thereof is not relatively complicated, and the ring is also applicable to the cap of small diameter when the container is inclined and the content is poured.

In the structure of the cap and the container mouth part in which a tamperproof ring connected to a lower end of a circumferential wall of a cap body of the cap by a plurality of brittle connection pieces is detached from the cap body by the engagement of a ratchet provided on a lower outer circumferential surface of the container mouth part with a ratchet provided on an inner circumferential surface of the tamperproof ring when the container is unsealed, a detachmentproof ring 122 is provided on an adjacent upper portion of the ratchet 121 on the inner circumferential surface of the tamperproof ring 120 of the cap 100, and a locking ring 222 of the outside diameter larger than the inside diameter of the detachmentproof ring of the tamperproof ring of the cap is provided below a male screw 221 of the outer circumferential surface of the container mouth part 200 and above the facing position of the detachmentproof ring of the cap.