PURPOSE: To easily put a cap and disable wrong opening for fully functioning sealing ability by making constitution of a cap body with an elastic hook which cab can opened/closed and a removable preventing piece formed by curling the upper end of a sealing member body inward after the cap is housed in the sealing member body.

CONSTITUTION: Since a cap 18 is set so that removable of a cap body 2 in closed state is impossible with an engaging protruding piece 12 of a removable preventing ring 13 of a sealing member 9 and a removal preventing piece 16, an elastic hook 5 can be elastically engaged with a base cap 4 by placing the cap 18 slantly on a base cap 4 of a vessel 3 with oil or paint filled and pushing the cap 18. When the cap 18 is to be opened, a driver or the like is used to cut a cut part 11 of a sealing member 9 and by removing a sealing member body 10 of the sealing member 9, the elastic hook 5 can be released and removed with the upper wall 6 of the cap body 2 pressed.