PURPOSE: To form a rubber plug of silicone rubber as base rubber and to make this rubber plug usable as a rubber plug of a plastic container for infusion.

CONSTITUTION: The rubber plug of a desired shape is molded by using the silicone rubber. The rubber plug made of the silicone rubber is good in resealability, eluate, coring and needle prick resistance. However, pull-out of the rubber plug arises. The molded rubber plug is, thereupon, subjected to a surface treatment. A strong acid soln. or strong alkali soln. is used for the treatment. A sulfuric acid, nitric acid and hydrochloric acid are suitable for the strong acid and a sodium hydroxide and potassium hydroxide are suitable for the strong alkali. The rubber plug is immersed into the soln. thereof and is taken out after 5 to 60 minutes. The rubber plug is washed with water and dried. The surface of the rubber plug having a slippery feel is then changed to a dry feel. As a result, the pull-out of the rubber plug made of the silicone rubber does not arise any more.