A piston configured to be engaged inside a cylindrical container includes: a sliding member having an upper fluidtight portion and a lower fluidtight portion; and a support member connected to the sliding member and having concentric flexions for urging the sliding member against the inner wall of the cylindrical container. The upper and lower fluidtight portions are each constituted by at least one annular convex bump portion having a thickness which is the greatest at a point where the annular convex bump portion is in contact with the inner wall of the cylinder.