A liquid ejection apparatus, comprising: a tank (9, 101) configured to contain a liquid; a liquid ejection head communicated with the tank (9, 101) and configured to eject the liquid; a casing (7, 31, 103) configured to place the tank (9, 101) and the liquid ejection head inside thereof and having an optically transparent area; and a cover configured to cover the casing (7, 31), the tank (9, 101) comprising a container portion configured to contain the liquid, an air introducing path arranged to introduce the air into the container portion, an inlet port configured to pour the liquid into the container portion, a sealing member configured to seal the inlet port, and an outlet port formed to supply the liquid through a tube into the liquid ejection head, and in an attitude that the liquid is ejectable from the liquid ejection head, the liquid may be contained in the container portion such that a liquid level of the liquid in the container portion is located at a higher position than a nozzle of the liquid ejection head in a vertical direction; an inlet port that is an opening of the air introducing path on a container portion side thereof may be located at a lower position than the nozzle of the liquid ejection head in the vertical direction; characterised in that the sealing member being covered by part of the cover, the container portion being extended in a first direction that is a longitudinal direction of the container portion, a dimension of the container portion in the first direction being longer than a dimension of the container portion in a direction perpendicular to the first direction, and at least a part of an area of the container portion of the tank (9, 101) overlapping the optically transparent area of the casing (7, 31, 103), having optical transparency along the first direction. a tank (9, 101) configured to contain a liquid; a liquid ejection head communicated with the tank (9, 101) and configured to eject the liquid; a casing (7, 31, 103) configured to place the tank (9, 101) and the liquid ejection head inside thereof and having an optically transparent area; and a cover configured to cover the casing (7, 31), the tank (9, 101) comprising a container portion configured to contain the liquid, an air introducing path arranged to introduce the air into the container portion, an inlet port configured to pour the liquid into the container portion, a sealing member configured to seal the inlet port, and an outlet port formed to supply the liquid through a tube into the liquid ejection head, and in an attitude that the liquid is ejectable from the liquid ejection head, the liquid may be contained in the container portion such that a liquid level of the liquid in the container portion is located at a higher position than a nozzle of the liquid ejection head in a vertical direction; an inlet port that is an opening of the air introducing path on a container portion side thereof may be located at a lower position than the nozzle of the liquid ejection head in the vertical direction; characterised in that the sealing member being covered by part of the cover, the container portion being extended in a first direction that is a longitudinal direction of the container portion, a dimension of the container portion in the first direction being longer than a dimension of the container portion in a direction perpendicular to the first direction, and at least a part of an area of the container portion of the tank (9, 101) overlapping the optically transparent area of the casing (7, 31, 103), having optical transparency along the first direction.