To assure a virginity of a commodity and enable an opened state to be clearly discriminated in reference to its outer appearance.

There is provided a stopper 1 in which it is projected at a paper container 20 for liquid, a seal opening cylinder 300 is movably stored in it, this is comprised of a spout 100 sealed with a sealing film and a cap 200, when the cap is rotated and removed, a seal opening cylinder is moved downwardly through a sliding contact between a guiding slant surface 241 of the cap and a guiding slant surface 301 of the seal opening cylinder and the sealing film is cut by a seal opening blade 310. The lower end of the circumferential wall 110 of the cap having a threaded part at an inner circumferential surface is provided with an tamperproof ring 230 having a plurality of ratchets 231 at the inner surface thereof, its inner side is cooperatively provided with a plurality of thin-walled ribs 223 through a ring-like slit line 222 and an outer circumferential surface at a lower part of the discharging cylinder 110 of the spout is correspondingly provided with a ratchet 112 fitted to a stopper protrusion at the tamperproof ring.