A new sealing device (30) which has not a capability of the push-through head portion (19) being re-inserted into the socket portion (2) again after the sealing condition has once been broken and further, a new sealing device (30) can be provided in which the evidence showing the fact that the sealed condition of the sealing device had been broken, intentionally, remains visible on the sealing device. The sealing device (30) is provided with a filament portion (3), a push-through head portion (19) provided at least at one end of the filament portion (3) and having a suitable connecting portion (23) which comprises, for example, a hooked portion, and a socket portion (2) provided at another end of the filament portion (3) and having a hole (10) through which the push-through head portion (19) can pass in one direction but not be withdrawn in the opposite direction, and wherein a stretch strength of the push-through head portion (19) is set at a certain level in that it is smaller than that of the filament portion (3).