Abstract: An air-permeable plug member of a rubbery material useful as a stopper of the opening in a vessel, for example, for the culture of microorganisms and a method for the preparation thereof, by which the air permeability of the plug member can be freely controlled, are described. The plug member is characterized by the unique cellular structure consisting of a combination of a closed-cell structure and an open-cell structure in an appropriate proportion. Such a plug member can be prepared by first preparing a foamed rubber body consisting of a closed-cell structure only and then subjecting the same to a foam-breaking treatment by passing the body through a gap between, for example, a pair of rollers of which the gap width is 10 to 50% of the thickness or diameter of the body so that the cell walls of the closed-cell structure are partly broken to produce an open-cell structure.