The invention relates to a plugging device (1) that comprises a base (10) secured around the neck (2) and a cap (20) supported by the base and capable of movement between a closed and an open position. In order to improve the tamper-proof characteristic of the device, the invention comprises using, on the one hand, a breakable thread (30) having at least one longitudinal end permanently connected to a first portion (211) of the cap or the base and extending, before breaking, lengthwise along the periphery of said portion and, on the other end, a lug (40) permanently connected to another portion, respectively, of the base or the cap, and protruding towards the outside from said second portion. The lug is adapted to press at least on the free part of the thread until the thread is broken at a breaking area along the thread when the cap is moved for the first time from the closed position to the open position thereof, and to be interposed between the edges of said breaking area when the cap is then placed back into its closed position.