In each of radial grooves 52 which are formed on a sealing disc portion 34 of a packing 10, low strength portions 64 are formed within a predetermined range from the center of the sealing disc portion 34 by decreasing the thickness of each of thin portions 62 by deeply cutting each of the radial grooves 52. When the center of the sealing disc portion 34 is pressed by the pressing means, splits are formed at the low strength portions 64, and extend to curved grooves 54 and circumferential grooves 56. As a result, it is possible to obtain a plugging member in which a draining port can be opened wide with a small amount of pressing force.