PROBLEM TO BE SOLVED: To provide a tamperproof device for cap whose satisfactory feeling in use is ensured while preventing rattlings of a tamperproof ring after severance at connecting pieces, and the state of opening of the cap can be discerned certainly. SOLUTION: The tamperproof ring 6 has a structure wherein an upper part 63 in the inner diameter slightly smaller than the outer diameter of a holding protuberance 33 at the lower part of a shoulder part 31 of a bottle main body 1 is formed at the upper end thereof. A ring main body 62 in the inner diameter slightly larger than the outer diameter of the holding protuberance 33 and in a height capable of contacting at its lower end with the lower part 32a of an indent 32 when the upper part 63 of the ring is held at the holding protuberance 33 is formed approximately in an L shape in the vertical section at the end of the upper part 63 of the ring. Elastic holding pieces 7 each deformative in turning up toward the inner wall surface 64 of the ring main body 62 and capable of riding across the holding protuberance 33 when being turned up toward the inner wall surface 64 are integrally formed at the lower end of the ring main body 62, and the elastic holding pieces are joined in elastic fitting with the elasticity thereof to the reversely tapered surface 34 of the indent 32.
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