Tail plug and writing instrument

A tail plug to be attached to a tail end of a shaft part of a writing instrument, the tail plug comprising:

• an inner plug that is fitted into the shaft part:
• and
• an outer plug that is coupled with the inner plug;
• the inner plug having a first opening through which a fluid can flow in an axial direction of the shaft part;
• the outer plug having a second opening through which a fluid can flow in the axial direction of the shaft part,
• at least one of the first opening and the second opening being configured to be closed when the outer plug and the inner plug are coupled with each other, wherein
• the outer plug comprises a closing protrusion for closing the first opening of the inner plug,
• the inner plug has a bottomed cylindrical shape with a bottom part and a cylindrical part, in which the bottom part has the first opening, and the closing protrusion of the outer plug extends in the axial direction of the shaft part,
• a tip end portion of the closing protrusion is configured to close the first opening of the inner plug when the outer plug and the inner plug are coupled with each other,
• the outer plug comprises a head part that is located at a rear end of the tail plug and that has the second opening, and a circumferential wall part that is fitted into the cylindrical part of the inner plug, and
• the circumferential wall part and the closing protrusion together form therebetween a gap enabling fluid communication with the second opening.