A plug in accordance with one aspect of the present invention is a plug for a container for containing a liquid, the plug comprising a plug body, a lid adapted to engage the plug body, and a columnar elastic body contained in the plug body and pierceable with an extraction needle; the plug body having an elastic body storage part for storing the elastic body, a liquid guide path communicating with the elastic body storage part, a plurality of axially extending longitudinal ribs formed on an inner wall surface facing the elastic body storage part, and axially extending groove parts formed between the plurality of longitudinal ribs, while a circle passing vertexes of the plurality of longitudinal ribs has a diameter smaller than that of the elastic body.