Abstract: An assembly of a tube having a breakaway plug mounted on one opening thereof is provided. The plug includes a hollow cylindrical segment fixedly secured to the tube, a closing segment, and a frangible annular portion located in the hollow segment apart from the secured portion. The plug is formed of material having a Shore A hardness of 68-76 such that the plug can be torn off at the frangible portion by twisting. Since the plug of relatively flexible material having a Shore A hardness of 68 to 76 is attached to the tubular body, the gas tightness and intimate contact therebetween are substantially improved to minimize the risk of the plug being accidentally ruptured by an external force when the tubular body is not on use, while maintaining the ability of breakage of the plug when the tubular body is on use.Preferably, there is provided a protector with such a hardness that it may not prevent breakage of the plug at the frangible portion for safeguard and easy breakage of the plug.