PURPOSE: To prevent contamination of medicine due to oozing of an adhesiveness promoting agent having sublimated into the medicine in a vessel or stagnating in the air stratum over the vessel by laminating the body of a rubber plug over the polyethylene sheet processing surface having a specific mean molecular weight whose one side is subjected to corona discharge processing, and giving a specific value to the surface tension of the sheet processing surface.

CONSTITUTION: Polyethylene sheet 3 material to be laminated over the body 1 of rubber plug shall have a mean molecular weight of 130-800 millions. The density of this polyethylene shall range 0.930-1.000g/cm2 while the melting point is 134-137°C. That surface of super-high molecular weight polyethylene sheet which contacts the rubber is subjected to corona discharge processing with an electric power of 5-35W.min/m2, and the surface tension of the sheet shall be 35dyne/cm. In the mouth part of the rubber plug 1 and its part corresponding to the surface of the medicine contacting part, molded polyethylene sheet 3 is loaded in a lower die with the corona discharge processed surface facing up. Thereover unvulcanized rubber is put fully in the cavity including the recess of the die, and the unvulcanized rubber is pinched between this and the upper die, and then heating by pressure is made from above with the upper die which has a recess in the form of a truncated cone.