To solve the problem of a liquid supply unit of the related art in that liquid supply may become difficult.The invention provides a tank capable of supplying a liquid to a liquid ejection head, the tank including: a liquid containing portion capable of containing the liquid; and an atmospheric air introducing portion constituting an atmospheric air flow channel capable of introducing atmospheric air into the liquid containing portion, wherein the atmospheric air introducing portion includes a buffer chamber capable of containing the atmospheric air, a liquid retaining member is housed in the buffer chamber, and in a state in which the liquid retaining member is housed in the buffer chamber, a gap that allows the atmospheric air to move between a first communication port and a second communication port of the atmospheric air flow channel is provided within the buffer chamber.