To provide a plug with a built-in pilferproof unsealing edge, which has a function for preventing the illegal opening and a good set property during capping process and separates completely a pilferproof ring from the cap, which is prevents illegal opening, from the cap when the cap is unscrewed.

A cylindrical body for opening, which is incorporated in a spout and having the opening edge at the lower end, descends and breaks a lower sealing film, and then opens a container, when the cap is unscrewed. The lower edge of the peripheral wall 220 of the cap 200 is connected with the ring 230 for preventing the illegal opening, having a fitting part 231 on the inner periphery at the lower end, through a plurality of thin walled weakened bridges 223 to be cut when the cap is unscrewed. A fitting part 112, which the ring for preventing the illegal opening of the cap is fitted into when the cap is unscrewed, is formed below the threaded engagement part 111 of the injection cylinder 110 of the spout 100. A stepped part is formed at a position located 20-60% lower than the height H form the lower end of the ring for preventing the illegal opening of the cap. The radius of the lower outer periphery of the threaded engagement part 111 is smaller than that of the upper outer periphery of the threaded engagement part 111 by 20-70% of T which is the thickness of the upper part thereof.