The sealing device 100 of a container 1 is provided with an inner plug 10 and an upper closure 20. The inner plug 10 has a separation part 12 for forming an opening by separation. A first screwing part 51 for attachment of the upper closure 20 is provided between the inner plug 10 and the upper closure 20. The upper closure 20 has an inner cylinder part 20c which is a support part catching a separation part 12 by a second screwing part 52 in opening the plug and separating the separation part 12 from the inner plug 10. The second screwing part 52 is in a reverse thread relation with respect to the first screwing part 51 and has a barb-shape which facilitates pushing the separation part 12 into the inner cylinder part 20c rather than withdrawing the separation part from the inner cylinder part 20c.