A rubber plug (1) for a vial container in which a discoid cap portion and a leg portion are solidly bonded by a two-step molding method is provided.In a rubber plug (1) for a vial container (2), a discoid cap portion (1A) is solidly bonded to a leg portion (1B) when the discoid cap portion (1A) is press molded integral with the leg portion (1B) by a two-step molding method. The bonding surface between the leg portion (1B) and the discoid cap portion (1A) is formed as a concentric concavo-convex surface having a boss portion (1F), a ring-like groove (1G) having a larger diameter and a ring-like groove (1H) having a smaller diameter. The leg portion (1B) including the bonding surface is formed as the same material of butyl rubber as the discoid cap portion 1A. The rubber plug (1) for a vial container (2) is suitable to a rubber plug for a vial container for storing liquid medicinal chemicals for freezing preservation.
