Abstract: A plug for a container capable of improving air-tightness and liquid-tightness is provided. A plug includes a first split piece having a cylindrical first fitting portion to be mounted on an opening portion of a container body and fitted to an opening portion space of the opening portion, an end wall which blocks one end of the first fitting portion, and a first flange portion extending outwardly in a radial direction of the first fitting portion from the other end thereof; and a second split piece having a second fitting portion which is fitted to the first fitting portion of the first split piece and forms a closed space between the end wall and the second fitting portion and a second flange portion extending outwardly in a radial direction of the second fitting portion therefrom.