A tail plug to be attached to a tail end of a shaft part of a writing instrument includes an inner plug that is fitted into the shaft part and an outer plug that is coupled with the inner plug. The inner plug has a first opening through which a fluid can flow in an axial direction of the shaft part; the outer plug has a second opening through which a fluid can flow in the axial direction of the shaft part, and at least one of the first opening and the second opening are configured to be closed when the outer plug and the inner plug are coupled with each other.