An air composition adjustment device for adjusting composition of air in a target space, comprising: a humidifier for humidifying a target space; and a composition adjuster (40, 60) that includes a separating unit (41, 61) that separates, from air-to-be-treated, supply air having a composition different from a composition of the air-to-be-treated, wherein the humidifier comprises: an air path through which water-extraction air flows; a cooling unit that cools the water-extraction air in the air path; a water extracting unit that extracts water that is generated when the water-extraction air is cooled by the cooling unit in the air path; and a humidifying unit that humidifies the target space by using the water extracted by the water extracting unit, wherein the air path includes a main path (221, 222, 263) that communicates with the target space and a discharge path configured to discharge a part or all of the water-extraction air to the outside of the target space, wherein the main path of the air path includes a path through which the air-to-be-treated flows to the separating unit (41, 61) and a path through which the supply air generated in the separating unit (41, 61) flows to the target space, wherein the cooling unit is configured to cool and dehumidify the air-to-be-treated or the supply air in the air path, wherein the water extracting unit is configured to extract water generated by cooling the air-to-be-treated or the supply air that flows through the air path, and wherein the humidifying unit is configured to humidify the target space by using the water extracted by the water extracting unit. a humidifier for humidifying a target space; and a composition adjuster (40, 60) that includes a separating unit (41, 61) that separates, from air-to-be-treated, supply air having a composition different from a composition of the air-to-be-treated, wherein the humidifier comprises: an air path through which water-extraction air flows; a cooling unit that cools the water-extraction air in the air path; a water extracting unit that extracts water that is generated when the water-extraction air is cooled by the cooling unit in the air path; and a humidifying unit that humidifies the target space by using the water extracted by the water extracting unit, an air path through which water-extraction air flows; a cooling unit that cools the water-extraction air in the air path; a water extracting unit that extracts water that is generated when the water-extraction air is cooled by the cooling unit in the air path; and a humidifying unit that humidifies the target space by using the water extracted by the water extracting unit, wherein the air path includes a main path (221, 222, 263) that communicates with the target space and a discharge path configured to discharge a part or all of the water-extraction air to the outside of the target space, wherein the main path of the air path includes a path through which the air-to-be-treated flows to the separating unit (41, 61) and a path through which the supply air generated in the separating unit (41, 61) flows to the target space, wherein the cooling unit is configured to cool and dehumidify the air-to-be-treated or the supply air in the air path, wherein the water extracting unit is configured to extract water generated by cooling the air-to-be-treated or the supply air that flows through the air path, and wherein the humidifying unit is configured to humidify the target space by using the water extracted by the water extracting unit.