A liquid ejection device of a first aspect of the invention includes a sub tank capable of fluid communicating with the main tank removably mountable to the liquid ejection device. The sub tank is configured to satisfy the formula: SH−SL≦MH−MN where SH is a liquid amount in the sub tank in a balanced state at which the liquid has flowed into the sub tank by a hydraulic head pressure on mounting the main tank with the liquid fully filled; SL is a liquid amount in the sub tank in a state where the liquid level is positioned at the outlet of the sub tank; MH is a liquid amount in the main tank when the liquid is fully filled in the main tank; and MN is a liquid amount in the main tank in a state where the liquid level is positioned at a threshold level.