has an engaging hook 84 formed on the other distal end thereof in contrast to the sealing flange 82 and a slit 85 extending from a free end of the directing rod 81 to a certain depth to allow the engaging hook 84 to deform when necessary when the locking mechanism of the preferred embodiment of the present invention is in assembly it is noted that the airtight ring 6 is received in the positioning space 83 of the core 8 and the sealing ring 2 is received in the collar 47 of the nozzle seat 4 so that after the collar 47 is extended into the cavity 14 of the cap 1 the sealing ring 2 ensures that the engagement between the cap 1 and the nozzle seat 4 is airtight before an airtight engagement between the ledge 52 of the membrane member 5 and the flange 45 of the nozzle seat 4 the positioning element 3 is movably received between the limiting element 12 and the driving element 13 of the cap 1 after the recovery element 7 preferably a spring is mounted around the directing rod 81 the directing rod 81 is extended into the hole 32 of the positioning.