PROBLEM TO BE SOLVED: To improve handleability without extremely roughening a surface of a rubber plug using vulcanized rubber with high stickiness.SOLUTION: A rubber plug includes a leg part that is inserted and fitted into a vial mouth part, and a shade part that is brought into contact with an upper edge surface of the vial mouth part. The rubber plug is characterized as follows: the shade part and the leg part are formed of vulcanized rubber surface roughness Ra of a top surface part, which is a top surface of the shade part, is in the range of 0.3-4.5 &mum and scattered projections are provided on the top surface part.
