A rubber stopper for a vial comprising a body member 1 and a sheet of polyethylene 3 having an average molecular weight of 1,300,000 to 8,000,000, a surface being contacted with an inner surface of a mouth and a liquid medicine, or a top surface 1b of which is covered. The molecular chains of polyethylene are partially ramified by heat to be branched and partially crosslinked with a rubber of the body. Therefore, adhesion of them is high and a smoothness of the surface of the sheet portion is improved.